A word of warning about cheap thermostats. I had two Repti-temp 500 thermostats fail within a month. I have also had two BAH-1000sc thermostats fail within a month. I used them to control flexwatt under 10 gallon aquariums for feeder insects for my lizards. I never put more than 100 watts through them. I don't know if it is just my bad luck, but I would never put my animals on a cheap thermostat. Spring for a Johnson Controls or a Ranco at least for your snake.

As far as rheostats go, they work in a pinch, but they are a pain in the butt. Make sure to check the temps a few times a day the entire time it is in use. Get one at a home improvement store instead of a pet store. You will spend less and get much better quality.