There's some cheap cooler/warmers (using Peltier tech) on the market. Has anyone tried them for long sourdough ferments? My guess is that they are not aimed at people wanting reasonably accurate temperatures, just warm or cold, but they may be good enough for proofing.
I've seen posts about DIY proofers with aquarium heaters and thermo controllers.
So do a site search on those terms.
Yes I have used one as a cooler, it is a used small wine fridge, and I use it to retard fermentation, mostly so it will fit with my work schedule. I have never tested it to see how closely it maintains a temp, but my guess is that it is good enough for my purposes. One thing to remember is unlike a refrigerator, they do not cool as effectively, and depending on the insulation, and ambient temp, you may not be able to get it to certain temps.