I use bark chips. I work on one inch depth as a minimum. It allows the liquid bits of poop to drain to the floor, keeping it out of contact with the snake as much as possible until I have a chance to clean it up. Mine seem to know when I leave the house for uni - they clearly let rip about half an hour after I've performed morning Poop Patrol and gone out!
Also, I think I remember my UTH instructions saying not to cover it with more than an inch of substrate.