This is similar to a project I'm working on for my plated lizards.
I'm using different thicknesses of sheet foam like at your local home supply center. It comes in thicknesses from 1/2" up to 2". It's real easy to work and sculpt with a wood rasp and/or a surform plane/file.
I'm coating it with slurries of mason's mortar with different colors of mortar color, but colored tile grout would work also. It would be easy to embed real chunks of rock/gravel too.
If you want an easy to clean surface you can spray it with flat polyurethane. This would make it washable but still give the dull rock finish.
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BigT
There is a difference between ignorance and stupidity. The ignorant can be taught, stupidity is beyond our control.
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