I have a stopper/bung with two holes and one hole has a thermowell about 12 inches long through the hole, so I just snake the probe down to the bottom of the thermowell and the other hole gets the airlock. It may not be perfect, but it registers from the center of the fermenting wort, about 12" down from the top of the bung...and seems to be a reasonable location. I am sure that there are currents and temperature differentials of some measurable degree, but I just live with what I have and it seems to work pretty ok.