My dad built pretty well what you're thinking on an old 5th wheel trailer frame. He put an RV rubber roof skin on the roof for water proofing. He had painted plywood on the walls outside but eventually he bought rv siding to put over the plywood, watertight and has worked well for over 10 years. For your application I might consider some type of metal roofing material as well. If you were going to have a shipping container originally, if you used colored metal building cladding on the outside plywood you'd have waterproof and no need to paint or anything down the road. wouldn't look much different texture wise tha a sea can. some other options for you. I like the way you're thinking.