During the winter I use a cheap space heater inside my chest freezer for a temperature-controlled fermentation chamber; I keep my house pretty cold (60F), too cold for ale fermentations. I figure I already paid for a big insulated box so I might as well make use of it! To power the space heater I ran an extension cord through the freezer’s drain hole.
The adjustment knob on the heater doesn’t have any numbers, just a “hotness” scale, so I’m labeling the dial with what each position gets me. To record the ambient temp I filled a beer bottle with water and stuck a thermometer inside; the water acts as buffer for rapid temp changes and also better simulates what the temp might be inside the carboy.