Just thought I would share that we ended up creating a fabulous tomb scene using about 5 4x8 sheets of the cheap white foam used for insulating. Broke some pieces in various geometric shapes and then broke off all sharp artificial looking edges and then roughed up flat surface by scrapping a knife across the surfaces.

We taped some large sheets together on back with duct tape and carved a large entry hole in the middle. After carving the various shapes we puzzled them back on and spray glued them to the base form, we reinforced some with wooden meat skewers forced in at angles (we were going to be moving this piece so it need the extra support).

We then spray painted it using regular water-based spray paint in two shades close to each other. The whole thing was sprayed first with the light color and then each stone was spray painted around the edges with the darker color.

Then we tucked in a bit of moss and a few plants along the top level, painted the inside black and we were done. It turned out great and if someone needs a photo to visualize, I will be glad to send one.

Thanks for the help!