Reefkoi had these sweet clam racks on sale at MACNA, and let me be the first to say it, what took you so long?  How many clams have we lost to them falling over, being too confined, getting overgrown, and getting too much sand on their mantle?  If I were running a store that sold clams, these things would be mandatory with each purchase.  In the same way that you must put a collar on your dog, you must put your clam on one of these unless you know exactly how to properly place it in an aquarium.  Check out Reefkoi for details.