Here's an idea: start with a shallow plastic or glass pan. You can pick something like that up at Wal-Mart. Maybe a plastic tub, if you want something particularly large or deep. That would make a good place to start.

I was going to suggest some stuff called "Great Stuff" - it's a spray foam. But I'm not sure it would be the best thing for water applications. But concrete or cement would make a great alternative for that.

You can also get "natural" stone-colored paints from your local hardware or chain department store. Let it dry real good (couple of days) before putting in the enclosure. Wouldn't want any of those harmful chemicals getting into the poor snakes systems.