Do you need to account for the CO2 pressure? Or is that just one of those where the fridge will burp itself through the seal once enough pressure builds up? I am curious as I am looking at probably building one soon to better control temperature, at least for the summer time (winter the basement is a pretty good and pretty constant 66F all winter long).
The CO2 should not pose any problems
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If you can wire a household outlet, get an STC-1000 for about $18-19 on Amazon. It's a dual temperature controller. You wire it into a standard 2-plug outlet that you mount in some kind of project box (either homemade or bought at Radio Shack). One (cool) outlet is for the freezer/fridge. Into the other (warm) outlet (this applies only if your fermenter chamber sits where it's cold like a garage in winter) you plug some kind of small heater and then put that heater inside the freezer.
Set the target temp (in Celsius) on the STC-1000. Set the tolerance (default is +/-0.5*C). When the temp (as read by the sensor) climbs 0.5*C above the target, it powers up the cool outlet and keeps it energized until the temp drops to the target and then turns it off. Likewise with the warm outlet if it gets 0.5*C too cool. You tape the sensor on the side of the fermenter and place some kind of insulation like bubble wrap over top of it so that it reads the bucket temp and not the air.
I'm borderline simple when it comes to DIY handyman skills but I was able to wire up a unit like the one in the video BigFloyd posted without electrocuting myself or burning down the house. I've been using it for over a year now with chest freezer and a heat belt I bought at my LHBS. Works great.
I am a terrible DIY guy, but I had no problem building a controller using the STC like others. For the warm outlet I used flex watt heat tape, the stuff they use in reptile cases and it works like a charm.