Thanks for the pics AF, I was going for the 1/4" because I want to put a kayak rack on the roof and wanted to be sure it was strong enough to handle the static and dynamic loads when moving. In another thread I got some help on the strength of 1/8" vs. 1/4" - the weight penalty seems worth it to me for the extra insurance.
My first take on the actual sidewall construction was the outer layer of plywood laid flat, then 1x3 wood used to create a perimeter, and door and roof rack supports. These would be glued and screwed to the ply. Then 3/4" rigid foam cut to fit in the open areas between the 1x3's, also completely glued to the plywood. Then a full coverage of glue on that and the second layer of ply laid on top and screwed down to complete the "sandwich".
One question on your insulation: Is that the white foam w/ silver foil on both sides? I saw that tonight at Lowe's and was concerned how well the foil was adhered to the foam - in my store it wasn't, it just peeled off easily, intact. I was thinking of just white or blue board rigid foam so it could have the best chance to bond to the wood.