Your tank over all might be a lot cooler than you think. With the prob in-between the aspen and the glass the surface temperature might be 3-4 (or more) degrees cooler. In my boa's cage with 2-3 inches of cypress mulch the thermometer I had between the glass and the substrate read around 100 to make an 88 degree hot spot.

Temp guns aren't too expensive (I got mine for around 20) and will give you an accurate reading of the surface temp. In my experience snakes are like rocks in the sun and absorb heat fairly well. As long as your hot spot is right I think it's okay that the cool side isn't perfect. They will still have the heat gradient they need. If you can get it up that's great, but don't break your back trying to get it absolutely perfect.

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