The following is just my option regarding a small scale home made incubator. I'm 14 hatched out of 24 duds\\failures and I am currently working on my next 24 batch.
I've tried both a water heater thermostat and a wafer thermostat.. and while i am more of a fan of the wafer, I think the most important element is the heat sync(s). I found one of those old coolers with the flat "water bottle" you could freeze and attach to the lid.. well, that's the bottom of my home made incubator. As long at the overall average of the air is 99/100 range, the water syncs will (if properly placed) make of for a lot. Having the fan blowing over this allows for the temp to immediately returned to normal after opening the lid for turning the eggs.