...the corrugated kind where it's got two layers and an air gap in between. Not the kind your cereal boxes are made out of. I'll spare everyone the thermodynamic explanation of why corrugated works a LOT better than regular cereal box cardboard (hint: the air gap), but try a layer or two of it.
Here's what I do: I put down a piece of cardboard that's a little bigger than the UTH, and then I put a sheet that fits the tank exactly above that. I wrap the exact fitting piece in paper towel, and just have to pull out that piece along with the paper towels for the weekly cleaning. It's easy, and cheap.
If you don't have corrugated cardboard around the house, you can pick it up at the grocery store for free. They're always happy to get rid of boxes. Especially discount grocers who leave boxes out for people to use instead of bags. Nobody every takes the huge boxes they put out, so just grab one of those.
Hopefully a layer or two should do the trick. If it doesn't, stack a little more. I have a UTH that was getting my tank up around 105-110, but now it's comfortably around 88-90.
Hope this helps.
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Blaine
0.0.1 HY/Patternless/Normal Leo (Echo)