Since I use tubs for QT and most get bumped into PVC cages when that's done, I have a variety of tub sizes and heat mat sizes I have had to use.
I don't honestly stick anything to the QT tubs themselves.. I want to be able to adjust sizes based on species and size of animals I get so that they have their needs met as goal #1. I don't just keep BPs like some do, so I have adapted things a bit to suit me.
What I do instead is use 1/2" pink insulation foam (sold in big sheets at the hardware store) and have it cut to the size of the table I use in my QT room, which happens for be 4'x2' *this piece is reused.
Beneath where the heat mat/pad goes on my designated hot side of the foam, I put the thermostat probe into a groove I made with a small knife and tape the cord down just below the probe, but not covering it.
Then I tape down the heat mat or heat tape with aluminum tape over the probe. I have a few different ones, bigger and smaller, that I choose between depending how big the tub I need to heat is.
So in the end, the heat mat is secured tightly over the probe, on an insulating surface so that heat isn't lost out the bottom nor can the probe read the temperature of the table rather than the heat source (which would be cooler in most cases and could cause the temperature of the heat mat to spike).
Then I just place a tub over top and line it up to have the heat under 1/3 to 1/2 the tub. This allows me to avoid fiddling with cables to be extra thorough cleaning my QT tubs, I can swap out the tubs for growing snakes or larger/smaller species, and the probe is still in position to read accurately, directly from the heat source.
Same sort of foam could be used beneath glass tanks, especially when someone taped down the heat mat without realizing the need for a thermostat.
As with any heat setup using thermostats, you have to do some testing to dial in the right temp for inside the tub before any animal is placed inside.
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