I already tried aspen, but it doesn't make a lot of difference.

Actually my thermostat also tells me the current temperature, so I only put this one. But I think I'm going to try like you said, and adjust the thermostat to get the right temp on the substrate. It'll make a really thin layer, I hope is ok.

And, just to know, has anyone tried not putting any substrate at all ? Just the snake laying on the glass ? Maybe it doesn't make a big difference with newspaper, as soon as it's hot enough...
Just asking, I won't do that.