I agree with you Shao
I would not want to put water in that tank until all of it is touching the stand.
I used a rubber similar to neoprene, wet suit material. Hard to compress with your fingers. It has holes in it like a sponge, but stronger.
Saw it on a work site and they were installing it between the windows and high rise. Comes in different widths and thicknesses.
I used it under my tank.
I know it will be a pain to instal, under the tank, heavy and awkward, but as target mentioned, "it will compress where needed."
Here is some info
https://canada.foambymail.com/neoprene-rubber.html
I would look for something like this
https://www.amazon.ca/Neoprene-Dualp.../dp/B07WZH14C5
but 1/4" thick by whatever width you need