Forced air. Uncalibrated digital Accurite thermometer/hydro combination sensor* at 100 with incubator thermostat set to 99.8. I wrap the sides of the incubator, so the air vent on top isn't covered.
*I'm using one of those fancy indoor outdoor weather stations for my thermometer. I just stuck the outdoor sensor inside the incubator. I was nervous about trying to calibrate the thermometer since it's got built in bluetooth signal stuff all wrapped up in there. The Accurite brand sensor and the thermostat on the bator were perfectly synced on trial runs. They could be spit on or happen to be off by exactly the same amount.
I use one of those too. It was never calibrated but through trial and error, I have excellent hatches by keeping it at 100°
