I appreciate cheap. I made a box out of foam board, caulk, and silver tape from Home Depot. Big enough to house 2 fermenters and a case of beer. I just rotated ice blocks (frozen 2L soda bottles work great) to maintain the temperature where I wanted it. With that amount of mass, the temp stayed pretty stable. I would rotate typically 2 bottles twice a day in the beginning of fermentation, then it would taper off when fermentation slowed. By the end of 2 weeks, maybe one 2L a day. This would all depend on the target temperature and the ambient temperature. My garage has an HVAC unit, so the ambient stays between 68 and 74F. Had no trouble keeping the fermenter in the 40's for brewing a lager. Just not effective for cold crashing, but I never worried too much about that. All in, less than $100 for the box.