Well there's a big difference between "opening a lid" and "opening a door." It's not a great idea to keep opening a fermentation chamber with a door- of course all the cold air will spill out. After 12-24 hours I'll peek in to see if there are signs of fermentation beginning and if I see it I don't open the chamber for a week- sometime longer.
My "probe" hangs about halfway down the chamber wall supported by a piece of Styrofoam. I have a thermistor attached to the end of a long thick copper wire inserted into a dry plastic vial. This long wire measures the ambient temp of the chamber next to the fermenter and hangs off the lid of the fermenter in the pic. The fan stirs the air around keeping convection going and blows directly on both probes.
So far when I measure the temp of the beer with my super-fast Thermapen it matches the temp of the surrounding chamber even at the height of ferment.