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You could try coco husk. They sell it under names like "forest bedding". It comes in a dry block and you mix it with water until it comes apart to a peat moss consistency. It's very good at holding alot of water. He shouldn't mind this on the bottom as it would be somewhat flat and unobtrusive.

I use moss in my moist hide and because it is very fibrous and tends to fall apart a lot, my moist hide is half a coconut on top of an overturned lid to a margarine container. This way the moss inside stays inside instead of getting all over my repti-carpet, but also, it keeps the water I spray on it from leaching out under the edges of the hide.