I figured out what I'm going to use on the outside of the case. I'll brush fiberglass resin onto thick cardboard and then glue a thin sheet of styrene on top. It'll give the sides the strength of 1/8 inch styrene but without the cost. I'm ordering all my styrene from US Plastics. The problem with ordering 1/8 inch styrene from them is that it has to be shipped motor freight. And with motor freight shipping you won't know how much it costs until it arrives at your door. My guess would be that it's fairly expensive.