My first carboy warmer device consisted of a cooler (that I already had) filled with water, the carboy itself, and an aquarium heater. The one I got was about $15.00 IIRC, and bottoms out around 70F. It works beautifully and automatically maintains that temp no matter what the surroundings. Even with the 6G carboy sticking out above the cooler almost halfway, it still maintains temp. I've also seen this done in rubbermaid storage containers with are very cheap.