I use a sheet of foam board from Lowe's that's 4'x8' and .5". It goes on the outside of the tank and I just cut most of the way through with a knife, then fold it so it breaks on the line.
You only want a little substrate over the uth because it won't heat through much. So that I don't have issues with humidity, I put more on the cool side and just barely cover the uth side. You won't crack the glass unless you set the thermostat really high and then pour water on it. Just having it slowly get warm won't be a problem.
I didn't have any trouble with a che on top of my screen. The bars are plastic in all the exoterra cages I've seen. If the lamp is touching the plastic and gets too hot it could melt a bit but if you're only using it for ambient temperature then it probably won't get too hot.
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