I'm currently using a small heating device to keep my fermentation fridge warm.
I wired 4 10 Watt 100 Ohm resistors in series using standard 120 Volt wall outlet. This will produce 36 watts of power.
With 8 watts each, those little resistors get very hot (like touching a light bulb), and could easily melt through the fridge.
My office just got rid of tons of old computers, so I pried out some old cpus with hefty heat sinks, and clamped the heat sinks onto the resistors. Event though these just get warm, I surrounded them with ceramic asbestos insulation just to be sure.
To control temperature right now, I'm just using an a cheap electrical timer. I plan on switching to a simple heat thermostat, but since it only measures the air temp, and I want to control beer temp...I haven't installed it yet. It is in the garage, so I check it to & from work, and adjust as needed.
So far, this system has worked quite well, and cost ~$10.