>>7246120Im that guy, and the answer is insulation foam, the exact stuff this guy posted
>>7246125I recommend using this stuff and not normal polystyrene, it cuts much neater and takes paint well, and is much denser so no crumbling off. I use the 2 inch sheets for the cliffs, and half-inch for stuff like the walls in this pic. The paint is just cheap acrylic paint from Michael's, I painted it black then layered on grey and a lighter grey to give it some depth. . You can cut the sheets with a knife, or (my recommendation) get a hot wire cutter. It melts thru the foam like butter and the only byproduct is water vapor, no toxic fumes to kill ya.
