Thanks for the responces.
My stand in an open top, designed to support the tank only on the outer rim on all sides. The tanks bottom is approx 3/4" above the outer supporting rim of the stand, so I don't know how useful a "sheet of anything would be.
I was thinking of trying the stuff below. It's a dense, 3/8" thick closed cell foam rubber self adhesive strip. It's the same width as the supporting frame of the tank, so I thought I would stick it to the bottom rim of the tank, then set the tank on the stand and level the stand. Should work?
I think so, as long as you are careful that there isn't any overlap or large gaps, though I think I would want to use something a bit wider then the frame edges, just to prevent it turning/buckling or something (may not be an issue, I don't know).
As to the sheet of styrofoam bit, yeah, in a typical glass tank, most of the sheet isn't used at all, I just don't like dealing with trying to piece together an outline, even though that is all that's really supporting the tank.
I've also heard of people recommending various foams for their insulating properties, but I think it's rather negligible unless you have the rest of the tank covered in foam as well.