The mirror box holds the mirror and its sell, its made from 15mm ply. I used 4 for the sides and a 5th piece (which has a 20½" hole in for the light path). This helps square the sides (the mirror cell does this for the other end of the box). Although for added strength I placed braces in each of the 4 corners. The inside of the box was painted with matt black floor paint (if its hard enough to walk on it should be OK for the box!)
Mirror Box. Underneath of mirror box with mirror cell in place.
Mirror Box. Front view of mirror box, with a 20½" hole, you can also see one of the braces in the bottom left of the box.
Mirror Box. Back view of completed mirror box and mirror cell.