We use clam shell one, sand is cheap from bunnings. We also put a brick on top of the top shell so it doesn't blow off, def don't want cats in there. We left it off one night and had to throw all the sand.
Never had a prob with mouldy sand, its more often dry, maybe thats due to Melb not getting enuff rain, lol.
Like Bron I hate it too, such a pain, as much as you try to brush sand off before they come inside I still always find it on the floors, on the couches.