Make the Side Panels
Making the side panels for the box is similar to making the back panel. Lay out three more pieces of the composite material, 2' long. Attach one short piece to one end of the longer pieces. Next, three pieces of composite material, each about 30" long, are laid diagonally across the three composite pieces. Draw marks to cut this diagonal piece to fit on the side panel. Cut the angles and screw the diagonal board into place. Next, using the diagonal piece as a guide, cut the excess off of the three composite pieces. You should now have a triangular side piece. To make the other side piece, follow the same steps. Next, attach the diagonal piece in the other direction. Cut off any excess and you should end up with two side pieces, each the mirror image of the other.