Mini fridges work great as they are already perfectly insulated. I used the one I describe in that link because it was the easiest for me to get at the time and I felt good about the fact that the proper humidity could be provided by a cooler full of water. It worked perfectly and I had no denting of the eggs until they pipped. Best of luck and feel free to contact me with questions. By the way, that incubator cost me about $40 total to make.

Jamie