I've used all kinds of material. Flat rocks, wood (branches or precut), PVC piping, metal tubes, plastic tubes, etc. I usaully go with what looks good as the base to compliment the model. I've drilled holes in stones glued in a tube and stuck the other end were the sun don't shine on the model. Even done one using just a heavy paperweight and brass tubing. Options are as limited as you're imagination.