I made a lid out of foam (it's four layers) and kept the old lid to use the cooler as a cooler when it's not a fermentation chamber. I add a water bath and a couple of frozen water bottles to keep it cool. I exchange the bottles every day, but that's all I have to do. I float a thermometer in the water, to judge temperature.
In the winter, if I have to heat it (rare), I add a $10 aquarium heater to the water bath and leave out the frozen water bottles.