Another suggestion would be to spend the $20 bucks (cheaper than a t-stat) and buy an Under Tank Heater. Try hooking that up to your rheostat and watching temps. It may keep you from having to get a thermostat.

If, however, you intend on adding more snakes in the future, i.e. more tubs, then I might be inclined to splurge now and buy the thermostat.

Hope that helps,