the orange box store carrys 2 inch pink stuff. And they will order it if they dont have it in stock. runs $28 a sheet.. took me 3 sheets to build my sides - 11 feet - from point to point. ( doing a mini hopper style ) wood is planned just for around doors, around the back hatch, and possibly the galley counter - not sure yet- as well as a canvas back hatch instead of a solid one.. so many ideas.
my goal is to keep it light. Plywood is heavy stuff. a 1 inch sheet is 80 lbs.. so the walls and roof alone on what I am building is 400 lbs.. before the needed framing and any cabinets. meanwhile.. 2 inch thick sheet of foamular weighs 10 lbs, giving me a weight for the same area of 60lbs. A bit of additional research is what got me hooked on the foam idea. the pink stuff has a compression rating of 60 psi. which means if you apply 60 lbs to a inch square area. its compresses by 5%.. which is insanely small . a test of a gorilla glued joint sold my other half last weekend. I made up a test piece to do some work with. He Broke it just to see what happened. ( welders seem to need to break joints for some reason).. the foam gave before the joint did, which impressed him ( again its a welder thing and I'm not gonna try to understand).