Well I think you already touched on the solution:

Make all of the cages uniform and get a new thermostat. Maybe use paper towels and a couple of hides (I use clay flower pot saucers with an opening chipped out because it's heavy).

Here's some food for thought. Heating across a UTH is not always uniform. The temp can vary by a lot across the surface. I imagine heat tape has similar issues.

Oh, but before you lose the thermostat - Sometimes it helps to keep an air gap between the heating element and the thermostat sensor. If the sensor is right on top of the heating element, the thermostat will turn on the juice when its too cold and crank the temp up. The UTH will heat the cage really fast and the temp will overshoot. The thermostat will switch off and the temp will drop rapidly until the thermostat clicks on again. So the temp will constantly swing up and down.

If there is an air gap between the UTH and the cage, it takes longer to raise the temperature of the cage. It creates kind of a lag time that gives the thermostat a chance to more precisely control the temp without overshooting. And it makes it harder for the UTH to heat the cage so it has to run longer and will provide more even heating.

Also also - if you have an air gap between the UTH and the cages, the cages will probably heat more evenly.

Get some little stick-on feet for the cages to raise them up 1/4" or so.

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