If you want to put something under, just use something with a broad footprint. Don't know how much point load the bottom can take.
Most maunufacturers went legless with the FVIR, because for the replacement market, maintaining approximately the same overall height is important to a lot of buyers, for ease of installation.
Rheem, possbily others, had to take a couple of gallons out of their ultra low nox models, for the same reason. A 50 is now 48 gallon, 40 is 38, etc.