I recently made a diaper bag for my niece and used the plastic material that companies are making their signs out of. It's a honeycombed pattern between two thin sheets of plastic, much like cardboard, and will hold up to anything. Being in a diaper bag I was afraid something might spill and ruin cardboard or foam board. I made a slip sleeve for it so it can be removed for washing. Found some at an art supply store. Didn't check a hardware store.