I've made rocks with urethane spray-on
insulation.
For big rocks,
build a rough frame from 2x4s, then
cover it with 1/4"
luan, small-hole
hardware cloth (screening), wadded newspapers,
etc. When you have a shape you like, have a commercial
insulation installer
cover it with foam. Two minutes later, you'll have a rock.
For medium-sized rocks, just cut out a rock shape on a piece of plywood and have the
insulation guy spray it. Be sure to concentrate on some areas more than others to give it some dimension. This takes communication because they practice to get their application smooth....just the opposite if what you want.
For small ones, take the spray head off the urethane applicator and let the foam dribble out like shaving cream into a pile.
For tiny ones, use blobs of Great Stuff (expanding foam in a can).
I made another rock (actually Christ's tomb for a Biblical
play) by carving out a solid chunk of
EPS foam into a 10' high by 16' long mountain. That one broke into two pieces for transport.
To
cover the foam and make it harder to damage, Menard's has a rubberized coating intended to
cover foam
insulation on basement wall exteriors. They have paintable and trowelable versions that really toughen up the foam.
What ever you do, be sure to prime the foam before final paint. I didn't one time and the paint fell off just before the show!
Good luck.