If you can get an old clam shell large enough to hold your your clam it will keep it like a baby in a cradle. Eventually the clam should attach to this old clam shell and you can place it a new location, if you want to, much easier. If you cannot get a clam shell you might try a section of PVC pipe large enough to surround your clam and perhaps a flat piece of live rock under the clam. The PVC section will corral the clam, keeping it on the rock - which it should attach to - preventing it from falling over. After the clam attaches to the rock you can remove the PVC section. The PVC section should be about an inch or so sliced off a PVC pipe that is a little larger in diameter thatn the maximum size of your clam.