The non-Plexiglas panels are solid. Two are plywood sealed with varnish, and one is panel-wood because we lost one in a move. All the panels are held together with hinges at the angles and brackets. The base and top act like box lids holding it all in shape. The top is a metal mesh surface (so we could set heater dish fixtures directly on it) with wood frame to form the lid and the base is plywood sealed in varnish with a 4 inch or so raised edge. I had to cut the top and bottom to get through a townhouse staircase so they are no longer solid but now fold.
I finally found some old pics from when I built it. Will post separately in this thread.