Plasterboard alternative for a model?

I am making a model that requires me to shape, paint, and glue something that will represent small stone slabs (1/2" thick). I do not have access to power tools. I need to cut out unique shapes and then sand them down considerably to make a dome. I was thinking about plaster board but the material I need does not need to be that sturdy. I basically need something between plasterboard and styrofoam (if that makes sense). Any ideas would be appreciated.