Turkey roasters, aquarium heaters, etc. all sound like an awful lot of trouble. Temperature control is critical but not that critical. Even so, the developer is the only temperature-critical step, and three minutes goes by fast. I fill a Playmate cooler with hot water and after several dry runs, found a water bath temperature that maintains the 100F developer temperature through the duration. I repeat the test periodically as variables change -- ambient temperature, developer loss, etc. -- and adjust accordingly.

I'm not an expert -- just started doing this myself. But this is working well for me thus far...