A thermo stat will not work very well for the lamp. I don't even think it'll work at all.
What type of UTH are you using? If your not already using flexwatt then I suggest it as most pet store UTHs don't get very hot and can't work well with a thermostat because when you tell it to be 95F it can only go up to 90F etc.
Flexwatt is far more cheaper than any UTH but you'll probably have to buy foil tape for it to sit under the tank. It REQUIRES a thermostat, usually a digital one is best, but I've heard its ok to use the non-digital ones as long as your not using it for incubation.
You put the thermostat probe at the lowest place your snake could get to to get warmth. :]
So if he burrows, underneath the aspen, right above the UTH.