If the gap is that small, I think the standard practice of putting a piece of foam under the tank should work well enough.
-I also do this as a precautionary measure, even with stands that appear to be perfectly smooth and level. If it's a stand I'm building, I usually build in a lip/edge to cover the foam and the bottom trim of the tank. Otherwise, I try and find some black foam or something similar.
I've heard of people using cork (bulletin board type), laminate floor underlayment, yoga mats, and the old ugly standby of expanded polystyrene insulation sheets.
right now I'm using some sheets of craft foam ('Foamies' - I picked them up at a Michael's/Jo-anne's) for a 20XT, but they are too small to use with a bigger tank.