The main problem with DIY incubators is temperature control.
I have seen so many type of temp control, I can testify to PID. Proportional-integral-derivative controllers are cheep, cheaper than most temperature controllers you can by from any poultry supply house. Ebay is loaded with them. And you can purchase them with a thermocouple and relay, to control any type of heat source.
The advantage to a PID controller is that it learns. As the temperature swings up and down, it measures time against rate of rise and learns just how long to hold on the heat source. The temperature reading can be calibrated to within .5 degrees, thus maintaining accurate temperature control.
So, check out Ebay, search PID controller and take accurate control over your DIY incubator project.