Swamp Cooler. Basically a container filler with water and frozen bottles that help reduce the temperature. Once I hit a temperature I can usually hold it pretty steady only changing a frozen bottle once or twice a day. Right now I am maintaining around 60* for my Cali Common.
Buy a giant bucket at walmart for $10 and fill it with water. Add carboy. Add ice/hot water as needed to regulate temperature. Due to the high specific heat of water, it takes a lot of energy for the temperature to actually change, meaning it's easy to keep the temp constant.
I took off the hollow plastic lid, and made a new lid out of four layers of foam (like wall foam insulation from Home Depot). I fill it with water, and it has wheels so it can be moved easily. I add frozen water bottles as needed. I have a floating thermometer in there, to monitor the temperature.