This panel is 22"x 44", and took approximately 4 cans. Once it is cured you can sand it, or carve it to any shape you want. If you wanted it to look like rock you could use silicone and spread colored sand into it.
Could do what I did and foam it up, carve it down, use some acrylic paints to paint whatever parts you want to look like rocks, seal it with polyurethane, and then cocofibre any parts you want to be "dirt". Doesn't affect the weight at all, and it looks pretty good!
@The Wild One Re: coverage - it took me about 1.5 cans of Multipurpose black (very similar to pond/stone) to fully cover a 16x30 background. One can jammed, so I ended up opening 3 haha! My 36x18x36 took about 8 cans.