I would make a bowl for it by getting a small tupperware shallow flat bottom container that is slightly larger than the clam, put about 1/2" of sand at the bottom, and then put the clam in it. Make an indentation in the sand with your finger, place the clam in it by the side of the bowl, this will help keep the clam upright. After the clam is upright in the bowl, put the whole thing in the bottom of the tank and LEAVE IT ALONE. Once it grows larger at least double in size and be able to stand on it's own two feet ... LOL ... j/k you can take it out of the bowl and place it where you want to.
The thing with baby clam is they don't like to be move around too much, this will stress them out and they will die.