Nothing. I have used Reptariums with soft trays for the bottoms, and wipe out any messes or extra water with papertowels. For monthly cleanings, I hose the cages down with a weak bleach solution and rinse well.
My store-bought enclosures, of all sizes, use a standard PVC floor. I just tilt-back the baby enclosures by lifting the front-end an inch or so to get the mist water to drain out the back into drain pans. I drill a dozen or more small holes in my larger enclosures' bottoms to allow for drainage. Since the feeders are larger, they can't get through the drilled holes.