It really isn't any different whether it's a freezer or refrigerator. You need something to control temperature. A popular choice is an
Inkbird 308. It controls both a heat source and a cooling source, and uses a temperature probe on a long wire to monitor the temp.
You simply turn the freezer to as cold as it will go, plug the freezer into the Inkbird, plug the Inkbird into an outlet, attach the temp probe to the fermenter (more on that below), and you're good to go. The Inkbird will turn the freezer on and off as required.
Optionally you can also use a heat mat like a
Fermwrap or a
Reptile mat to add heat to the fermenter if necessary. Fermwrap is 40 watts, the Reptile mat is 21 watts. I have both, and unless the garage (where my ferm chambers are) gets very cold, the 21-watt matt is sufficient. Can be covered with a blanket or some such to direct the heat inward if necessary.
I always bump up the temp at the end of fermentation and the heat mat/wrap will do that. All I need to do is change the temp on the Inkbird and it'll take care of it.
If there's a downside to a freezer, it's the lifting of a fermenter into and out of. If that's not a concern for you, then it's fine. I prefer refrigerators for that reason.
Below is a pic showing how I use the temp probe. I take a piece of closed-cell foam and cut a small slot in it to accommodate the temp probe. I hold the probe/foam against the fermenter using a bungee cord. The foam insulates the probe against the ambient temp in the ferm chamber.
