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If you are making your boxes 16 inches (41 cm) wide, measure 16 inches (41 cm) from one side, mark it, and repeat this for each divider. This will ensure that you’re placing the dividers evenly so that all the boxes end up the same size. You should measure and mark along both lengthwise edges of the bottom piece to be sure your measurements are straight. Follow the markings you made on the bottom piece and glue your dividers in place, using a heavy object to prop them up on each side while they dry. Glue your side pieces in place on each short edge of the bottom piece and prop them up as well. When you’re propping up the piece to dry, use a level to make sure they’re positioned straight up and down. Otherwise, you’ll have trouble attaching the top piece. Once the glue on the sides and dividers is dry, take the support pieces you cut and screw or nail them down on either side of each divider. Do the same next to each side piece. It should fit on the boxes so that it’s flush with the top edges of each side and divider. Attach it with 2-3 screws or nails in each place where it meets a side or divider.  Make sure your nails or screws are long enough to go through the top piece and at least 1 inch (2.5 cm) into the sides or dividers underneath it. For extra stability, flip the boxes over and put 2-3 screws or nails in the bottom piece in each place where it meets a side or divider. The back side of the boxes, where you will attach the back piece, should be facing up so that you can rest the back piece on top while you attach it. Use at least 6-8 nails or screws for each box to ensure that it’s well secured. Since the bottom piece will stick out farther in the front, you may need to set the boxes on top of cinder-blocks so that it can lie flat. You can also have 2 people hold the back piece in place while you attach it instead of turning it over. These are the two long, thin boards that you will attach to the top front and bottom front of the boxes to secure them and keep bedding from falling out. Flip the boxes over so that the front faces up and the back piece is resting on the ground. Place the front lip along the bottom edge and attach it, using one screw or nail for every side and divider. Do the same with the trim along the top edge. Set it so that it runs along the front edge of the bottom piece where it sticks out further than the boxes. Attach it using 1 screw or nail for every box.

Summary:
Measure and mark where your dividers will go on the bottom piece. Glue your sides and dividers to the bottom piece using wood glue. Attach supports to the bottom piece next to each side and divider. Add the top piece to your boxes. Turn the boxes over and attach the back piece. Attach the front lip and trim. Add the perch board to the bottom piece.