Just aspen or newspaper is fine. The hot spot if kept at the proper (90-92) temps will not burn your snake so extra measures are not needed. As far as ambient temp in the room goes as long as the the cool side doesn't drop below 78 your fine.
Grab an accurite if you don't already have one. The unit itself will measure the cool/ambient temps along with humidity and the prob will measure your hot spot. (it's not great at exact temps but will get the job done and give u a ball park around 2-3 degrees +/-)
And hot side temps should measure the point on the glass where the heat pad is heating it, not above the substrate.
Thermostat prob should be between the heatpad and the bottom of the glass.