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You can buy this at a construction supply store like Lowe’s or Home Depot, but you can also use scraps from a place like Habitat for Humanity’s ReStore for much cheaper. If you have scrap wood lying around from a past project, this can be an ideal use for it. If you are making 16 × 16 × 16 in (41 × 41 × 41 cm) boxes, the scraps should be at least 16 inches (41 cm) square. You should also have 6 pieces that are as long as all your boxes put together, and 3 of them should be at least 16 inches (41 cm) wide. You should have 1 of these for every box you’re making, plus 1 more. They should be the width and length that you want each box to be.  If you’re making 16 × 16 × 16 in (41 × 41 × 41 cm) boxes, your side and divider pieces should be 16 inches (41 cm) square. Measure and mark where you will cut before using your saw. You should always wear work gloves, safety glasses, and a dust mask while cutting, and work on a stable surface. These should be the same length as your side and divider pieces, but only 2 inches (5.1 cm) wide. You will end up using these pieces to hold the sides and dividers in place and make the boxes more stable. Cut 1 for each side and 2 for each divider. The width of these pieces will be the same as your side and divider pieces, and you can calculate how long it should be by multiplying the width times the number of boxes you’re building. For instance, if you’re making 3 boxes that are 16 inches (41 cm) wide, your top and back pieces should be 16 inches (41 cm) wide and 48 inches (120 cm) long. The bottom piece will create a ledge on the front and give you a place to put a perch bar for your hens to hop up onto when they enter the boxes. You’ll want a short board that stretches across the bottom front of all the boxes, to keep the bedding and eggs from falling out. This should be as long as your top piece, and at least 3 inches (7.6 cm) wide. You can also use a long wire for the lip instead of a board. This will make it easier to clean your boxes as it can be easily removed, but may not be as effective at keeping eggs and bedding from falling out. The wire can be hooked onto a screw or nail on each side of the boxes, and removed when you want to clean dirty bedding materials out of the boxes. These should be as long as your top piece and 2–3 inches (5.1–7.6 cm) wide. One will go along the front top of your boxes and screw into the top and sides for extra stability, and one will be screwed to the front of the protruding bottom piece to serve as a perching bar for your chickens. This will give them something to grasp when they hop up onto the platform to enter their boxes.

Summary:
Choose a flat wood like plywood that is about 2 inches (5.1 cm) thick. Cut out your side and divider pieces using a circular saw or table saw. Make support pieces for your sides and dividers. Create back and top pieces long enough to cover all your boxes. Make a bottom piece as long as the top piece but 10 inches (25 cm) wider. Cut 1 long piece for the front lip. Cut two pieces for the top trim and perch bar.