My advice would be to look into balsa foam. It's pretty cheap, comes in blocks or sheets, and is kinda like floral foam. It's that porous and kinda gritty texture. I've done some basic things with it and if you draw out your rocks with a toothpick (scratching out the outline and such) and then scrub the surface with an old toothbrush it will smooth it out, like alcohol on sculpey. It would also easily glue to the cardboards and can be painted, it doesn't melt like styrofoam. Hope that helps!
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