If you are using a cooler then the easier methodology is to fill the bottom 15cm with water, get a recirculating pump, two aquarium heaters, a handful of flower pots, and a sheet of egg crate. Set the heaters to your temp, drop them in the water, put in the recirc pump, put the pots in and set the egg crate on top. Close the lid and let it run a couple days to get to temp. The heated water heats the air and acts as a heat sink to buffer temps when you open/close the lid. It also keeps humidity up, I just put the eggs in the hatch boxes dry and they stay plump and happy the whole time. Because the heaters have their own thermostats they keep the temp stable and having two means you have redundancy if one fails