If you're going to make it out of new materials you can get styrofoam in sheets at most DIY warehouses along with the adhesive for it. Unfortunately I haven't found an good way to melt and remold styrofoam. However, polystyrene when it's not in a foamed form is a decent building material. You can also get unexpanded styrofoam beads that you can mold yourself. Another solution is to get this stuff called A B foam that comes as two liquids you mix together. It foams up dramatically and hardens. Again you would have to build a mold. All that said, why not use wood?