I actually just picked up a single basin like that while I was out. It looks like it should just wedge in there tightly so hopefully I won't have to worry about putting in any sort of shelf to hold it up.

I've been reading that soil or sandy soil is the best, as opposed to just playsand. I have some eco earth type stuff soaking right now, and I do have play sand here. I was planning on just going with straight eco earth but I could easily mix some sand in there as well.