Im not positive on if the ceramic would be safe for the tank chemicaly, but im sure it would be fine if its a clam thats not to be in the sand. Im can't remember which ones do better in sand or on the rocks.
you could go to Home Depot or lows and get one depending on the size of the clam, if its small you could even just put it on a 4x4 tile and when it gets bigger glue that tile to a bigger one.