If you "must" have some sort of substrate, you can use sphagnum moss. or pure organic dirt, or sand. Just know that impaction is of greater concern with dirt and even more so with sand. With sand it has to do with the jagged edges of the individual crystals making up the sand you see. They tend to get caught in the intestinal wall. As far as I know sphagnum moss is the safest. it is digestible, and your cham would literally have to be eating it by the mouthful to risk impaction.
But for cleanliness sake....bare bottom is best.
And of course there are feeders to consider. Crickets, super worms, and most other feeders will burrow in or hide in substrate.