I just had a thought -- instead of foam core, which is pretty thin and not all that durable, what about some of that sheathing stuff they use to insulate houses? I think it's basically like styrofoam, covered in some kind of tough plastic, and it's thicker, like 1/2 - 3/4". Seems like you could glue/pin it together like foam core, but it would be sturdier and hold more weight. Then you could cover it with whatever you wanted. Just brainstorming here...... :)