When I built Brigadoon, l lucked into finding some discarded styrofoam that had been used to float houseboats or docks on. Because it had been left outside, it was very rough and great for the rocky hills. I actually painted rock walls which looked right by comparison. But check out local docks to see if you can find any of this stuff. The biggest bits were 4ft square and about 1 1/2 ft deep. I've used them several times since then. They were heavy at first because of the water that had gotten logged in them, but they've dried out over the years.