Thermostat. That's what you need. Go to the herp supply websites and search around. I use a herpstat and I'd recommend it to anyone. Set-it-and-forget-it kind of thing.
Some things to keep in mind -
1. You get what you pay for. Expect to pay at least $100 for a good one. Pet stores sell some on/off products that are a little "iffy" in my opinion.
2. Proportional is more accurate than on/off. You described a thermostat that turns something on and off. But they make ones that make small incremental adjustments to keep the temperature dead-on.
3. Rheostats need constant adjustment, but with a thermostat you're covered even if the air temperature in your house changes.