In doing control systems many years ago, there was a company called Omega that had a set of manuals that showed you how to make about any temperature control circuit you can imagine with their modular pieces. It would take a themocouple mounted in a strategic location, connected to one of their simple units, then use the output of the unit to trigger a relay to turn on the heater. No big deal.

I have been in software for a long time now, but I'll bet a search for Omega Temperature Control would home you into a wealth of information.

Good luck,