Perhaps part of the problem is that these Zilla thermostats have huge probes. At least they used to back when I used one many years ago. They are so big and round they would lift a big part of the UTH away from the tank of you would put it between the tank and the UTH.
Back then I put the probe under the UTH and under that would be a sheet of 1 inch Styrofoam that the tank sat on. I made a small indentation into the Styrofoam so that the probe had space but was still firmly touching the UTH without going anywhere. And everything stayed level despite the big probe. The weight of the tank kept everything in place nicely. The probe was also taped in place.
Don't know if I'm explaining that well..
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