Step 4: Insulate the Back
Now we will make a frame to hold the foam and insulation board on the back of the panel. Cut your frame material into the same dimensions as the main frame, using the 2' sections made from the 1x2's (gives an extra 1" for the corrugated tin to fit) and longer sections with the 1x3's. Build the frame the same as you did for the main one too. Secure the frame in place with wood strips cut from the leftovers of the 1x2's and 1x3's, nailed/screwed to the side of the main wooden frame, and then nailed/screwed into the sides of the new frame. To seal the ends, use two or the foam insulation strips (the ones that fit the tin's corrugation).
Now the fun: use your spray foam (the stuff for "Big Gaps" is probably best here) and go back and forth along the troughs in the corrugation on this side. One can will not come close to filling the box, so use it mainly to fill the troughs and act as glue to hold a slab of foam board insulation.