In my incubator I get a fluctuation of less than 1/4 of a degree. Mine is made from a very well insulated wine fridge. The difference really is in the thermostat. My herpstat keeps it so tight because it controls the amount of power going to the heat tape (which it displays it on the thermostat). it feathers it slightly up and down to keep it spot on. If I open the door and the temp drops a degree or two it powers to 100% then slowly feathers it lower till it lands right on my setting. Make sure your window you cut is sealed tight and then get a good thermostat.