Flexwatt and a thermostat. The glass door is a plus I think. I am using as freezer that I am installing flexwatt and a ranco thermostat in, along with some fans, but I wish I had a glass door so I wouldn't have to let out the heat to check on the eggs. I may have that figured out tho.
I had a friend who used a Ranco on his incubator. Good thing he woke up in the middle of the night, for some odd reason, to discover it had failed and hi temps were up at 103.