Sure, last winter, I had tried one. Personally, I used it only once, but it could work fine. My seedling mat cover the beg, but 1". That's where I inserted the probe. Then this was inserted in a neoprene bag. I collected 4 Heineken bags with handles that came with mini kegs. The same material the picobrew company uses for the brewing sleeve, but in bag format, and green. You set the target temp on the PID and make sure the temperature probe is centred on height of the keg, and wedged between the neoprene and the keg. I spot checked with a light thermometer, and the strip thermometer on the keg, all 3 matched close enough. There's no heat you feel externally from this, so I don't see why you would not put that in a fridge as well. The seedling mat controller I used increases the temp by only 10 to 20 degrees. If the fridge is cool enough, you could FF a lager as well in theory. I don't seem to find any pictures, but worst case I can set it up if you'd like.