Hi, I just read your thread. Nice tank!
From that picture of the clam I'm thinking maxima, but did you have a look at the shell? Is it longer than it is tall or pretty close to the same height and length? The former would a maxima and latter a squamosa.
The sand bed is usually the best place to avoid falls unless you don't have enough light to allow it to be placed down there. I can't remember what kind of light that sunpod puts out so you or someone else would have to make that call.