I understand the need or desire to build a very low cost incubator, but when you think about it if the few extra dollars for a wafer thermostat is going to stand in the way of someone buying an incubator from you, not sure they should be in the chicken business. It is a very small price to pay in comparison to feed cost brooder cost, etc,etc. I cant imagine that there is a stable way to control temperture without the thermostat in there. For one thing the eggs (chicks) create their own body heat when they get to about 18 days so you will have an additional source of heat in the cooler.
Not sure it would be a responsible person that would set eggs in a incubator that doesnt have some sort of thermostat for regulating temp. It would be one thing if they either hatched or didn't but you have the possibility of deformed, weird, birds from bad temps. Rough navels, mushy chicks, badly feathered just to name a few. Those of us with the ability to hold temps within a degree have enough trouble
JMO