Heat Mat Issue

[Bogertophis]

You could use a piece of ceramic tile or something similar under the UTH. It would keep it in place as well as limit the amount of heat escaping below

That's the kind of thing I use for my glass tanks that are sitting on wood "aquarium stands" (open underneath tank) & even for those on furniture cabinets. As long as your
UTH has a little (required) "breathing room" (gaps so it doesn't over-heat) you can insulate under your tank most of the way to promote heat rising into the tank. Ceramic
tile is perfect for right under the UTH as it's a good "heat sink" & adds safety (not flammable). If you stop in a store that sells & installs flooring & ask them for a few extra
tiles, if they have leftovers from jobs they'll be cheap or free. Just measure ahead of time to make sure you get what fits your application. On my solid wood cabinets, they
are self-insulating but the tile underneath helps prevent excessive drying out of the wood from the constant UTH heat.