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?