Then I made a couple of prototype (I may make these from ¼ inch plywood later) side reflectors from cardboard. These are approximately 16 inches by 20 inches.
To attach the side reflectors you will need to make four hinges. If you have a ready-made box, this is the only part that requires any real construction, but even so, it only took me about 20 minutes. I made the hinges from eight little blocks of ¾ inch pine and 4 pieces of 5/16 inch dowel rod. The little blocks are 2 ½” to 3” wide and 4 inches long. I would make the reflector wing blocks a bit longer if I remake them as it would give more stability. Drill holes ½ inch from the end of each block and insert dowels in the four that go on the reflector wings. Glue/screw the female hinge blocks to the box. If you staple (as opposed to gluing) the reflector male hinge blocks onto the reflectors, they can be reused when the cardboard gets wet or breaks. When fastening the blocks to the wings leave plenty of slack so the wing does not bind in the primitive little hinge.