You could put the fermentation bottles in a bath of water chilled to the right temperature. That's what I plan to do, eventually.
An idea that occurred to me earlier today when I stopped to sip water from a water chiller at the grocery store was to buy an old chilled water fountain with a good working compressor, plumb it in to a temperature activated valve, then into a long tub or trough that will hold four to six carboys. To keep things simple, the trough could be drained by an overflow tube at 12 or 13 inch depth (about 2/3rd of the height of the carboy). Route the drain line to a low drain, like your shower, bathtub, or above the trap under your sink.
I'm not sure how cold a water fountain chiller will chill water, but you want to make it go as low as you can. They don't have a large capacity, I don't think. You'll be mixing in chilled water from the chiller with a larger mass of water in your trough or tank, but it still shouldn't take much flow to maintain a temp in the 60's.
You want the trough or tub that you make (think fiberglass over a plywood frame) to be just big enough to hold your carboys, insulate it well on the outside, and make an insulated cover for it so just the tops of the carboys poke through. Even if the tub is a close fit, there will be a large mass of water around the jugs that should be able to maintain temperature fairly well, especially if you insulate the heck out of it.
You want to get a submersible pool pump to circulate water in the tank so your temperature probe that is wired to the solenoid-controlled water valve gets a good average reading.
I wish I had the space to build something like this.
Another idea, by the way, and possibly a better one if you can find it cheap, is to buy a large refrigerator from a store that is going out of business. The ones that are used to keep flower or film cool would be ideal. You could keep it in your garage. A photography store near me sold two of its three large double glass door refridgerators that used to hold film because film sales had dropped off significantly since the advent and rising popularity of digital imagery. The owner said he sold them for $300 each to a guy who wanted to keep his wine in them. Each one would easily have held maybe five carboys on the floor of the unit, and would have had a large amount of chilled airspace above that for whatever you want to keep cool. I'm pretty sure, too, that they are deep enough to fit one of those 27 gallon conical stainless steel fermentation vessels. Wouldn't that be neat?
