5 DIY Projects to Defend Your Home From Invaders
In Defending Your Castle: Build Catapults, Crossbows, Moats, Bulletproof Shields, and more Defensive Devices, William Gurstelle poses the question: Using modern materials and construction techniques, could you successfully defend your home from history's greatest invaders? Gurstelle created 12 step-by-step DIY projects based on ancient artillery, and we've got five for you here.
1. Hudun Pao: The Crouching Tiger Catapult
While there’s nothing better for knocking down a wall than a catapult, Pak So and his soldiers showed how effectively catapults could be used for defending a castle as well.
The Hudun Pao, or Crouching Tiger, is one of the simplest catapults that you can make, but it’s powerful and accurate. One of its key characteristics is that the throwing arm rotates on a vertical plane until it smacks hard against the machine frame at the end of the swing. This arrangement means that your war engine packs a wallop. It also requires that you build the frame very solidly. This model can be easily scaled up or down to suit nearly any defensive or offensive job, from a science fair project to turning back attacking Mongol hordes.
A bit ironically, the Crouching Tiger was originally a Mongol design. The rotating throwing arm was powered by a falling weight, and it was so effective that it was adopted by many Chinese and Korean military engineers. Over time, they modified it to make it shoot farther and better, just as we are doing. Our modification uses elastic cords instead of a falling bucket of rocks for power.
The plans that follow detail the construction of a relatively small machine, capable of flinging 3-inch water balloons and other small projectiles across and even beyond the typical backyard.
In case of an actual Mongol attack, you could scale up this project by using larger hunks of wood and substituting large steel springs for the elastic cords.
- Safety glasses
- (2) 2-inch × 8-inch boards, 36 inches long: Sides
- (1) ¾-inch-thick plywood board, 10 inches × 24 inches: Stop
- Wood glue or staples
- (1) 6-inch × 6-inch foam pad
- (1) 1½-inch-diameter PVC pipe, 43 inches long: Lever Arm
- (2) Steel eye bolts ¼-inch diameter, 2-inch-long shaft (Eye bolts are threaded bolts with an eye at the end for attaching a hook.)
- (4) Nuts and washers
- (2) 1½-inch-diameter PVC pipes, 6 inches long: Pivot Arms
- (1) 1½-inch-diameter PVC tee fitting
- PVC cement and primer
- (6) 2½-inch-long deck screws
- (2) 2-inch × 8-inch boards, 12 inches long: Ends
- (3) 1½-inch-diameter PVC end cap fittings
- (1) #8 machine screw, 1½ inches long, nut, washer
- (3) Steel screw eyes, ¼-inch-diameter, approximately 2-inch-long shaft (Screw eyes are similar to eye bolts except the ends are woodscrews instead of threaded bolts.)
- (4) Elastic (bungee) cords with hooks, nominally 20 inches long
- (1) Trigger [see Step 11]
- Small water balloons or other small projectiles
- Threadlocker (e.g., Loctite)
- Table saw or wood saw and 3-inch-wide wood chisel
- 2-inch hole saw
- Electric drill with ¾-inch spade bit, #18 twist drill, and Phillips head screwdriver bit
1. Put on safety glasses. Use the table saw or wood saw and chisel to cut a ¾-inch wide by ½-inch deep straight groove into the face of each 36-inch-long board, 4 inches from one end. These boards will be the Sides of the frame.
2. Use the hole saw to cut a 2-inch-diameter hole in the face of both Side boards, about 12 inches from the end and 8 inches from the grooves, as shown in [below]. Drill two ¾-inch holes in the plywood board that will be the Stop, as shown in the diagram. (You will insert the bungee cords through these holes in a later step, so you may want to make sure the ends of the cords will pass through the hole. If not, use a larger drill bit.) The holes should be 2 inches from the top and 1.5 inches from the sides. Glue or staple the foam pad to the center of the top of the Stop.
3. Drill holes for the eye bolts in the 43-inch PVC Lever Arm at 17 and 19 inches from the end, as shown in diagram 2.6. Insert the eye bolts and fasten securely with washers and nuts. Be sure to position the bolt’s loop upward on the bolt closest to the firing pin end and position the bolt’s loop downward on the bolt closest to the Pivot Arms. Build the throwing arm assembly by connecting the Lever Arm, the Pivot Arms, and the tee fitting using the PVC primer and cement as shown in diagram 2.6. Do not attach end caps at this time. Read and follow the directions on the PVC primer and cement containers, including information on using it safely.
4. Line up the sides so the grooves face inboard and insert the Pivot Arms in the 2-inch holes. Insert the plywood board into the grooves as shown in diagram 2.5. Put the 12-inch boards along the open ends to create a box. Fasten the wood pieces together with the deck screws. If necessary, have a helper hold the pieces so you can use the electric drill and Phillips head screw bit to make the box.
5. Place the end caps on the Pivot Arms but do not use cement or primer. (If you do, you’ll never be able to take your catapult apart.)
6. Drill a #18 sized hole in the center of the remaining PVC end cap. Insert the #8 machine screw through the hole with the screw head on the inside of the cap. Using threadlocker, fasten it securely with nuts and washers. Use PVC primer and cement to attach the cap to the end of the Lever Arm.
7. Fasten 2 screw eyes to the top of the end piece closest to the Stop. Each screw eye should be 3½ inches from the corner of the box.
8. Fasten the remaining screw eye to the face of the end piece opposite the Stop, 2 inches from the top and 6 inches from the side, as shown in diagram 2.5. This is the trigger screw eye for step 10.
9. Connect the hooks on the bungee cord ends to the eye bolt on the Lever Arm, through the corresponding holes on the stop, and hook the remaining end through the screw eyes on the end.
10. All that remains is the trigger. You can build your own release or use an archer’s arrow release from an archery supply store (Internet search term: “archery arrow release”), a pelican hook from a sailing supply store, or a horse trainer’s panic snap from a tack shop (search term: “horse panic snap”). Attach one end of the selected trigger mechanism to the trigger screw eye.
Operating the Crouching Tiger
1. Carefully pull the throwing arm back. If the tension on the arm is too great or too little, you can add or remove bungee cords. Don’t go overboard, however, as too much stress could break the machine! Latch the lever arm to the archery release, pelican hook, or panic snap. (You may need to use a short loop of rope or a carabiner to attach the trigger release to the trigger screw eye depending on the size and shape of the trigger selected.)
2. Tie a small water balloon or other projectile to a string loop and place the loop over the firing pin.
3. Release the arm and watch your projectile fly!
1. Depending on the strength and number of bungee cords used, the lever arm can strike the stop with great force. Don’t skimp on padding! Make sure there is sufficient padding on the stop to prevent the arm from breaking when it hits. Also, don’t use too many bungee cords or stretch them excessively.
2. Keep hands, face, and other body parts well away from the plane of rotation of the throwing arm, especially when the machine is cocked and ready to fire. Be very careful of that rotating arm!
3. Do not stand in front or behind the plane of rotation of the arm. Use this machine outdoors, in areas where the projectile will do no harm.
4. As always for projects of this nature, wear safety glasses and use common sense.
2. Alexander’s Tortoise
You may think that it’s easy to build a battering ram. Well, you’re right. As far as weapons of war go, you won’t find one much simpler. Still, there are a few tricks that separate a really good battering ram builder from a mediocre one.
Note: Although this is a model, its dimensions and construction can be scaled up in case you need to take a real run at something.
- (4) 1-inch × 1-inch square wooden dowels, 2 inches long: Wheel Mounts
- Wood glue
- (4) 1-inch × 1-inch square wooden dowels, 6½ inches long: Short Horizontal Frames
- (4) 1-inch × 1-inch square wooden dowels, 8½ inches long: Long Horizontal Frames
- (4) 1-inch × 1-inch square wooden dowels, 3½ inches long, one end on each piece cut at 45 degrees: Rafters
- (1) 1-inch × 1-inch square wooden dowels, 10½ inches long: Ridge
- (4) 1-inch × 1-inch square wooden dowels, 4 inches long: Vertical Frames
- (1) Piece of leather or other heavy fabric, 10 inches × 11 inches: Cover
- (6 to 10) #0-sized grommets
- (1) 1-inch-diameter round hardwood dowel, 11½ inches long: Ram
- (2) 3/8-inch round wooden dowels, 3 inches long: Ram Supports
- (8) Small screw eyes (#212 or similar)
- (1) 1-inch or 7/8-inch metal-tipped furniture leg glide, nail-on style: Ram Head
- 1/8-inch-diameter cord, 3 feet long
- (4) Axle pins for wooden wheels (Available in craft stores)
- (4) 2-inch wooden wheels (Available in craft stores)
- Electric drill with 3/8-inch and ¼-inch bits
- Grommet setting kit
1. Drill holes for the axles (usually ¼-inch, but check axle diameter before drilling) in the 2-inch 1 × 1 inch dowels for Wheel Mounts, as shown in diagram 4.4.
2. Using the wood glue, construct the lower frame of the Tortoise as shown in the diagram. The Short Horizontal Frame pieces are the 6½-inch dowels, and the longer sides are the 8½-inch dowels. Position the Wheel Mounts in each corner, glue in place, and let dry.
3. Using glue, assemble the wooden pieces of the upper frame of the Tortoise. Just like the lower frame, the Short Horizontal Frame pieces are the 6½-inch dowels, and the longer sides are the 8½-inch dowels. The Rafters are made of the 3½-inch dowels with the 45-degree cuts placed on the frame pieces. The 10½-inch dowel fits in place at the top as a Ridge.
4. Place each 4-inch-long dowel piece in one corner of the lower frame as shown in diagram 4.6. Place the upper frame atop the pieces. Glue everything into place and let dry. Check glue label for drying times.
5. Attach grommets evenly spaced along the 10-inch-long sides of the leather piece as shown in the assembly diagram. Follow the directions on the grommet kit to securely add grommets.
6. Drill parallel 3/8-inch holes in the 11½-inch round oak dowel, approximately 2¾ inches from either end. Add a bit of glue and insert the 3/8-inch dowel into the hole so that the same length of wood extends on both sides of the large dowel.
7. Attach one small screw eye to each end of the 3/8-inch dowels. You may need to drill a small pilot hole to get the screw eye started depending on how hard the wood is. Next, attach the remaining four screw eyes to the upper frame, 2 inches from each end, as shown in diagram 4.6.
8. Hammer the metal-tipped furniture glide onto one end of the 1-inch dowel.
9. Attach the battering ram to the upper frame by tying the cord from each screw eye on the 3/8-inch dowel to the closest corresponding screw eye on the upper frame. Cut the cords so that the battering ram hangs approximately 1 inch above the lower frame when tied into place.
10. Using the cord, attach the leather covering to the top frame by looping the cord through the grommets and around the upper frame members in a continuous fashion. You can also use glue to attach the cover in addition to the cord.
11. Insert axles into the holes on the Wheel Mount pieces and attach the wheels to the axles with glue.
Congratulations, your battering ram is ready to take on castle walls.
Battering Ram Science
Once the ram-wielding soldiers chose a likely spot for their assault, they drew close to the target area. They grabbed ropes that were attached to the swinging ram and pulled back. As they pulled, the ram moved upward, like the hands moving from six o’clock to nine o’clock on a clock face. On their leader’s signal they let go and the ram crashed into the wall.
The amount of energy imparted on each blow of the ram is equal to the mass of the ram times the vertical height to which it was raised times a constant based on the acceleration of Earth’s gravity. The equation looks like this:
Energy of blow = mass of ram × height the ram is raised over its rest position × gravitational constant
So, to add more energy on each ram blow, the Macedonian soldiers could either add weight to the ram or pull back harder, thereby raising the ram higher on the arcing path made by the swing of the rope.
3. The Cheval-de-frise
The great thing about a model cheval-de-frise is that it is easily built with hand tools and can be scaled up to life-size using the same construction principles if desired. As described below, this model makes an interesting pencil holder/desk accessory and history-related DIY project, and one sure to get attention from visitors!
- (1) square wooden dowel, 1-inch × 1-inch × 10-inches long
- (18) round (not hexagonal) wooden pencils, sharpened (Mirado Black Warrior pencils are sold in many US office supply stores. If you can’t find any at your local store, use this Internet search term: “round pencils.”)
- Drill press (a hand drill will work, but it takes more care to make a perfectly straight hole) with a 5/16-inch drill bit
1. Using your ruler and pencil, carefully lay out the hole drilling pattern shown below on the dowel. Make a mark for each hole, laid out on the centerline of the long faces, with each hole 1 inch apart from the next parallel hole, and ½ inch apart from the adjoining perpendicular hole, as shown in the drilling diagram.
2. Drill 9 holes in each face of the dowel. Keep the drill bit perpendicular to the wood face and drill 5/16-inch diameter holes through the square dowel, according to the drilling pattern laid out in step 1.
3. Insert the sharpened pencils into the holes in the square dowel. You can easily adjust the position of the pencils (the spikes) up and down in their holes, and in so doing, change the angle at which they point. A 50-degree angle works well, but you can adjust the angle depending on what barbarian horde is attacking you. For example, if you are defending against, say, a Macedonian phalanx, then arrange the spikes so they point at a more acute angle from the ground. If defending against a human soldier versus Mongol pony, then tip the cheval-de-frise so the spikes point higher.
4. Carpini’s Crossbow
Designing and constructing a crossbow of military or sporting quality requires great skill in addition to fairly sophisticated metal and woodworking tools. However, with minimum tools and the directions that follow, you can make a model crossbow suitable for defending your castle against “model” (by this I mean imaginary) Huns or Mongols. You’re probably thinking that the model will not provide much in terms of actual defensive capabilities. And you’re right; it won’t. But the concepts used in building it could be expanded upon, and if you are clever, a crossbow with usable power and accuracy could be fabricated, should a real threat emerge.
The model crossbow is an excellent team project, and you’ll have a great deal of fun in both making it and using it. Remember, however, that you’re building a weapon that shoots a projectile; therefore, using it safely is the most important part of the project. Don’t aim at things you don’t want to shoot, don’t overstress the parts by pulling the bow back too much, and wear protective gear (for example, safety glasses) as appropriate.
Note on materials: You have quite a bit of latitude in building this project. You can make the stock, bow, and trigger a bit longer or shorter and it will still work OK. The dimensions in the drawings are the ones that worked best for me and provided good results. However, feel free to experiment with the dimensions and perhaps get even better performance!
- (1) 1¾-inch × 1¾-inch square dowel, 36 inches long: Barrel (available at most large home stores or lumberyards, or search online for “1¾ square dowel”)
- (1) 1¾-inch × 1¾-inch square dowel, 12 inches long: Gunstock
- Wood glue
- (2) Wood strips, 3/8-inch × ¼-inch × ½-inch: Spacers (You can either cut these to size from a piece of scrap pine board or buy a precut piece of basswood at a hobby store.)
- Aluminum strip, 1½ inches × 9 inches × 1/8 inch: Trigger
- (1) 1½-inch hinge, with mounting screws
- Wood strip 1½-inches × 2½-inches × 1/8-inch: Trigger Pad
- (1) Loose coil spring 9/16-inch diameter, about ½ inch long (You probably won’t find this exact size at the hardware store, but you can buy one a bit bigger and cut it down to size.)
- Axle grease (optional)
- (3) #8 short machine screws and nuts
- (1) 4-inch-diameter U-bolt, strap, and nuts: Stirrup
- (2) #10 screw hooks: Puller Hooks
- (1) Round dowel, 1-inch diameter × 6 inches long: Puller
- (1) 3/8-inch × 1¼-inch × 36-inch-long oak board: Bow (You can cut this yourself from a board or dowel, but it’s easier to buy a piece of moulding this size at the home store. It’s called a mullion. Home Depot’s Internet part number is 203116469.)
- (2) #8 wood screws, 1 inch long (for attaching Bow)
- (1) Piece of stiff, inelastic cord about 48 inches long: Bowstring (Four-ply waxed linen cord works very well and most fabric stores sell it. Hardware stores sell #18 mason twine, which works acceptably.)
- Small projectiles to fire
- (1) 3/8-inch-diameter bolt, 2 inches long: Firing Bolt
- Safety glasses
- Hand saw or table saw
- Sandpaper or file
- Router with a ¾-inch straight router bit and a 3/8-inch straight router bit (Alternatively, you could use a saw and chisel, but it’s more work.)
- Electric drill with 3/32-inch, ¾-inch wood bit
- Screwdrivers, pliers, vise
1. Put on safety glasses. Use the saw to cut one end of the 36-inch-long square dowel, which will be the Barrel, at a 15-degree angle, as shown in the diagram. Cut one end of the 12-inch square dowel, the Gunstock, at an angle as shown. Glue the pieces together with both flat ends flush and the angles toward the same direction, and let dry. Round the edges of the Gunstock with sandpaper or a file
2. To make the ammo groove in the Barrel, use a router or a saw and chisel to cut a ¾-inch groove, ¼-inch deep, in the center of the top face of the Barrel. This groove holds and guides the ammunition. See diagrams 2.10 and 2.11.
3. To make the Trigger groove, use the router or a saw and chisel to cut a 3/8-inch wide by 3/8-inch deep groove in the top face of the Barrel perpendicular to the ammo groove, at a point about 21 inches from the angled end. Drill a ½-inch-diameter hole in the exact center of the 3/8-inch groove. See diagram 2.12 for help in determining how the hole and grooves align.
4. Glue the 3/8-inch by ¼-inch by ½-inch centering spacer strips to the front edge of the Trigger groove, one on either side of the centered hole in the ammo groove, as shown in diagram 2.13. The centering spacer strips are important because they position the bowstring in the middle of the trigger groove so the bolt pushes against the center of the bowstring when the Trigger is pulled.
5. To make the Trigger, use a bench vise or the side of a sturdy table to bend the longer aluminum strip as shown in diagram 2.12. You will need to experiment a bit with the two angles on this piece of aluminum to obtain the action required to move the firing bolt upward when the Trigger is pressed. Luckily, aluminum bends easily and is quite forgiving, so you can make several adjustments if necessary. Take your time and experiment until the firing bolt smoothly and dependably lifts and fires the bowstring.
With the woodscrews that came with the hinge, attach one end of the hinge to the Barrel. Depending on the hinge you purchase, you may need to insert a thin wood spacer, made from a piece of scrap, between the hinge and the Barrel, to mount the hinge.
Place the spring around the 3/8-inch bolt and insert the bolt into the ½-inch hole in the Gunstock with the bolt head on the bottom. Make sure the bolt moves up and down easily in the hole. File or sand the interior of the hole and use axle grease if necessary.
Drill three holes in the aluminum using the hole pattern on the hinge as a pattern. Then, attach the Trigger to the hinge using three #8 short machine screws and nuts as shown in diagram 2.14. When you pull back on the long end of the Trigger, the short end should push the bolt up, which in turn will push the bowstring out of the Trigger groove and fire the projectile.
6. Make the cocking Stirrup by drilling a hole, ¾-inch or larger, depending on the diameter of the U-bolt you are using, in the Barrel perpendicular to the ammo groove, at a point about 1¼ inches from the angled end of the Barrel. Insert the U-bolt and secure with the strap and nuts. Make the Puller by attaching the screw hooks to the 1-inch-diameter dowel 2 inches from either end. Align the hooks so they face the same way. See diagram 2.15.
7. Use the router or saw to cut a string groove in both ends of the 3/8-inch by 1¼-inch by 36-inch-long oak mullion Bow as shown in the circled detail in diagram 2.9. Attach the bow to the angled end of the barrel with two #8 screws, 1 inch long. Position the screws so they avoid hitting the U-bolt. Tie two loops in the ends of the bowstring so that it is about 45 inches long. (You can adjust the length of the bowstring later, based on your results.)
Using Your Crossbow
Holding the crossbow so the front end is facing downward, place your foot in the stirrup to steady it. Then, using the puller, pull the bowstring back and down into the trigger groove. Place your ammunition (typically a small rock or arrow, but you can be creative with this) in the groove just in front of the trigger groove. Aim and then pull back on the trigger. Pulling back on the trigger will push the bolt up, which in turn will free the bowstring from the groove. The bowstring will push on whatever ammunition is placed in the ammo groove and propel it forward.
Inspect the crossbow parts for wear and replace as needed. Remember: use common sense when operating your model crossbow and do not aim at things you do not wish to shoot. (In case of actual Mongol attack, you may want to consider using a larger and more powerful method of defense!)
At the Battle of Liegnitz in 1241, the Mongols spread a gargantuan smoke screen across the field of battle. It was produced by burning now unknown items in a strange and demonic-looking smoke projector. Using clouds of smoke that both acted as an irritant and hid their movements, the Mongols’ Smoke Monster was a device that could turn the tide of battle.
According to medieval chronicler Jan Dlugosz, this early chemical weapon was a huge lance with a giant X painted on it. It is topped with an horrible, ugly head with a chin covered with hair. As the Mongols withdraw some hundred paces, the bearer of this standard begins violently shaking the great head, from which there suddenly bursts a cloud with a foul smell that envelopes the Poles and makes them all but faint, so that they are incapable of fighting. . . . Seeing that the all but victorious Poles are daunted by the cloud and its foul smell, the Mongols raise a great shout and return to the fray, scattering the Polish ranks that hitherto have held firm, and a huge slaughter ensues.
This is one of the first recorded uses of chemical warfare in Europe, and it was a turning point for the Mongols in their war. The monster threw Duke Henry’s army of eastern European knights into confusion as the “evil-smelling vapors and smoke” hid the activity of the Mongols from them. The Mongols charged through their opponent’s lines and made quick work of them. Contemporary reports say that Henry’s army took more than 30,000 casualties, many of whom were butchered after death; their ears were removed and sent back east in giant sacks to the Mongol’s main camp as a memento of the great victory. Duke Henry himself fared even worse. After he was killed in battle, his head was removed from his body, placed upon a spear, and carried around the walls of the city.
5. The Hidden Book Safe
Sometimes it’s better to be clever than strong. That’s frequently the case when it comes to safeguarding small valuables within your home. Despite your best efforts, it is possible that the barbarians will eventually make it into your castle. Perhaps you weren’t home to defend it or you beat a strategic retreat. But when you’ve got Huns in your home, it doesn’t matter how they got there, it only matters what happens next.
Barbarian invaders, be they Huns, Mongols, or anyone else, have no intention of sticking around for long after they’ve taken what they want. After plundering a place, they make quick work of leaving. Therefore, it makes sense to carefully hide valuable items so they are not discovered and stolen.
Various techniques for hiding valuables have been developed over the years. The book safe, in which a cavity is cut in the interior pages of an otherwise unremarkable book, is so well known that many pillagers are well aware of it. Still, it is tried and true, and because the average modern castle contains so many books, it’s unlikely that an invader will take time to examine each book for hidden valuables.
Choose a hardcover book, at least 1½ inches thick with a trim size of 5½ by 8 inches. While it’s not necessary, many book safe makers have a wry sense of irony and choose particular books, such as Adam Smith’s The Wealth of Nations to conceal cash, The Eustace Diamonds by Anthony Trollope to hide jewelry, or any Zane Grey novel to stash a handgun.
At first glance, cutting a cavity in the pages of a book seems so easy and straightforward that no directions are required. Armed with a sharp X-ACTO knife and plenty of spare blades, one can cut a reasonable stash area in any book. But it takes a lot of time this way, especially if the cavity is more than half an inch deep. In addition, using a utility knife to cut a few pages at a time invariably yields a rough and uneven cut. If you are patient and diligent it is certainly possible to make a fine book safe this way, but a rushed job will cause crinkled pages and may tip off the thief or plunderer that something is up with this book.
A faster, neater, and all-around better method makes use of a jigsaw, a plywood form, and clamps. The rigid form makes cutting a neat cavity simple and easy.
- Hardcover book, at least 1½ inches thick, 5½ × 8 inches
- (2) pieces 3/8-inch-thick plywood, cut to the same width and height as the book’s pages
- 1-inch paint brush
- White glue
- Heavy object, such as a brick or barbell weight
- (2) 4-inch C-clamps
- Electric drill with drill bit diameter just larger than the size of the jig saw blade
- (2) 5/16-inch bolts, an inch longer than the thickness of the book, with nuts and fender washers
- Jig saw
1. Use a marker to mark off the length and height of the cavity desired on one of the pieces of plywood.
2. Choose the appropriate number of pages for your cavity. If you select a large number of pages, the cavity will be deeper and you can hide larger items in the book safe. However, paper is surprisingly difficult to cut through, so some smaller jigsaws will not be able to do the job.
Place the plywood forms on either side of the selected pages. Clamp the paper and plywood forms together securely with the C-clamps. The rigid form makes it easy to cut a clean, uniform cavity.
3. Use the electric drill to drill pilot holes for the jigsaw blade in opposite corners of the cavity layout drawn on the plywood.
4. Drill 2 3/8-inch holes in the center of the plywood forms. Insert the 5/16-inch bolts, using fender washers on either side, and tighten the nut, as shown in figure 3.11.
5. Use the jigsaw to cut out the paper cavity.
6. With a brush, spread white glue on the interior of the cavity to prevent fraying. Place a heavy object on top so it dries flat.
Your book safe is done and ready to safeguard your valuables.