The way I see it it doesnt really matter what heat source you use. As long as the bulb isnt a spot light and doesnt directly heat and shine to the eggs youll be fine. You just need to have a constant 89-90 degrees in that container. You can have a room like mines that isnt air conditioned right and you can easily heat those eggs by putting them in the closet. But remember a constant 88-90 is needed no drop or changes!

You some kind of pulse- proportional thermostat. Thats a thermostat that reduces or increases the temp. Perfect way to keep that temp at a constant 90. Also buy a thermometer to measure the rooms temp that houses the container.