Step 4: Build the frame and assemble the panel
1. Cut one 2x3 to two lengths of 22.25" for the ends of the frame. Screw the other 2 2x3s into the ends to make a rectangular frame.
2. Wrap the transparent plastic around this frame and tape in place to make a transparent lid to fit over the collector. In my case this is for test purposes only, since I intend to eventually install the collector underneath transparent roofing material in my attic. The 22" panel width will fit between existing rafters which will provide a ready-made frame.
3. Cut the plywood to 24"x8'.
4. Cut the polystyrene sheet to 7'4" by 3'9" and place it centered on the plywood. This will be the insulation for the back of the panel.
5. Test-fit the collector and drill two holes in the plywood large enough for the hoses to easily fit through (about 1" should be good). Make one of these holes into a slot by drilling another hole right beside it and cutting away the wood between them. This is to allow for thermal expansion of the corrugated plastic sheet. Plastics typically have a high coefficient of thermal expansion. If you restrict the panel from expanding, it may warp and cause a leak.
6. Now stack the whole works together: First the plywood, then polystyrene, then the collector, then the transparent cover.
7. Secure the transparent cover to the plywood back with several clamps (or you can screw it on, but initially you might want to be able to remove it easily for access to the collector)