I just spent most of the summer, when I couldn't brew because of temps, trying to figure out the best way to create a chamber of some sort.
Here's what I wanted:
1) Needed room for 4 x 5 gallon carboys
2) Needed 10 gallons at a time at a specific temp with the other 10 gallons at another temp (so I can lager a 10 gallon batch and still run ales).
3) Needed to be cheap. I make pretty good beer without a fermentation chamber so justifying this means not blowing my budget. I had $150 from a beer competition win and I didn't want to run over that if I didn't have to.
What I had:
1) A pretty good amount of space. At least enough for a big chest freezer.
2) A basement so I wasn't too worried about looks.
3) Some scrap 2 x 4's for framing, but not enough plywood to line it.
4) 1/2" sheet insulation.
What I did:
The ultimate solution I came up with was quite simple... 2 dirt-cheap full-size refrigerators from craigslist and 2 of those cheap e-bay temp controllers. All in, I'm at just over $100 and I can easily fit 4x5 gallon carboys (2 controlled separately from the other 2).
Any other solution I could come up with, including using my scrap to make a chamber, was going to come out higher and/or not offer me the control of 2 carboys separate from the other 2. If you've got a small refrigerator already, maybe that is the way to go, but you can get a full size fridge on craigslist for almost nothing if you are patient and jump on a good deal when you see it. In fact, the mini-fridges are generally more expensive because landlords regularly want to just get rid of these things when tennants are evicted, go to jail, or otherwise leave junk behind that the incoming people don't want. I also looked in to chest freezers, but they are less common on craigslist (especially to hold 4 carboys and they generally command prices that matched the entirety of my budget (or way more new). I got my fridges for $25 and $50 and it really only took me a week of looking to find and get both. I know I could have done better than $50 if I was more patient.
Bonus for big fridges: As long as you make your temp controller easy to uninstall, you don't have to make permanent modifications to the fridge. That allows you to convert it back to a regular fridge/freezer when you aren't using it as a ferm chamber or to re-sell them, recovering a good amount of the money you spent to make them in the first place. Also, this makes them a fairly "ready-made" solution.
Small fridges with a constructed chamber will have trouble keeping you at lager temps without burning up the compressor, especially if the room it is in is not cooled in the summer (mine isn't). And, as far as efficiency with old fridges (or having multiple ones), since most of the time you aren't dropping the temperature that much (in the winter I'll actually be heating), I wasn't too worried about it. What I do know, though, is that the fridge manufacturers probably did a better job of insulating and sealing than I would be able to do on my own... especially without spending a bunch of money on materials to do a good job of it.
