Burn

[Coluber42]

If you are just using the thermostat to control the heat pad, the cheaper ones *should* be fine. They will allow a much larger temperature swing than the more expensive Herpstat, but they'll work for giving your snake some heat to lie on without letting it get too hot. The one caution about the cheaper ones is that if they fail, the heater goes all the way on rather than all the way off. So you might want to use a lamp dimmer in addition, just so you know for sure that full blast isn't actually full blast. But anyway, Hydrofarm thermostats are sold for starting seedlings. There's a ZooMed one that I think is similar. The expensive ones are Herpstat and Vivarium Electronics, which come in various versions depending on how many devices you want to control with it.

The thickness of the substrate above the heat mat is potentially sort of a complicated subject; thick substrate acts as an insulator, so that the surface of the glass underneath could get really very hot even with the substrate surface not that hot. But then if the snake digs down just a little further, it can theoretically encounter a temperature much higher than you would have thought. Lots of heat mats recommend no more than half an inch of substrate over the heat mat, for exactly this reason.

One last suggestion is that you prop the entire tank up on blocks so that there is air flow underneath. You'll lose more heat from the heat mat that way, so it's less efficient in that sense, but it will reduce the amount of possible heat buildup.

Another complicating factor is that the snake's body itself can have the same kind of insulating effect, meaning that the temperature isn't too high when you just check the surface by itself, but when the snake curls up on it, heat can build up. A thermostat will prevent that from happening.
You can also use a dimmer, if you can get it dialed in. This insulating effect is a little hard to test, but you can check it by using a thermometer with a probe (like those indoor/outdoor household digital thermometers) and put the probe over the hot spot. Then check what the temperature does when the snake is actually sitting on top of the probe and turn down the dimmer until it stays stable at the desired temperature while the snake is there. When the snake leaves the temp will drop, but that's OK.

I use aspen in my BP's tank, but over the hot spot there are a couple layers of paper towels instead. The water bowl and the hide hold them in place, and there's a little aspen around the edges. That lets me set the heat mat to really low power using a dimmer, so there's no possibility of it ever getting that hot enough to burn. I might have to reassess that particular setup when he gets big and strong enough to rearrange his furniture, but it's working fine for now. It's also handy for cleanup, since he likes to do his business in his warm hide, and it's easier to see the mess.