Yes it is nice to be able to put your projects INSIDE something isn't it? You know, to hold everything together so it doesn't flop around? Discuss the techniques here!
Styrofoam is weak, soft, and brittle. It wouldn't catch fire easily, but it would not be durable at all. Not to mention the huge mess that comes from working with it...
Use traditional portable-making materials and you'll be happier with the outcome.
What about that one product they were coming out with that you spray on styrofoam that makes it like concrete. They were going to use it to build houses out of styrofoam...
A case from just styrofoam, no. But you could make a case mold with styrofoam, then cover it with fiberglass. That would be pretty sweet. I've used Bondo fiberglass kits before, and they're pretty easy.