There are several well known brands of closed cell insulation board that have a nice flat surface and are tougher than Styrofoam. Yet they are only marginally heavier than Styrofoam, which is considered an open cell insulation. There is the blue Dow insulation board and the Owens Corning pink insulation board that comes in various thicknesses. They are made from extruded polystyrene, and it cuts nicely with a razor sharp utility knife. Then there is also the DuPont Tuff-R brand that is usually a bit more expensive. It is aluminum foil faced polyisocyanurate. Here up north, they are available at most lumber yards. Naturally, being a light foam material, the surface can be dented fairly easily. I've joined it to itself or other surfaces using spray foam insulation such as minimal expanding Great Stuff. If you wanted to get fancy, you could dress it up a bit and protect the edges from abuse by making a simple frame out of MDF or other molding.
