I see a lot of people build fermentation chambers out of foam board and then use a computer fan to circulate the air. I'm sure you can build one of these for $50 or so, but I've also seen many 5-7 cubic foot chest freezers go with $100 or less. I'm not talking about junk freezers either. I bought a like new, 12 foot one, for $100. Of course you still need a controller, but in the end you'll have great temperature control and you won't be filling up your food freezer with bottles of water.