I installed one on my kettle. Its not a great idea because high heats during grilling can over extend a dial therm. They have a spring inside and it can get uncalibrated by as much as 50* if the therm gets too hot.
what ever you do, don't make it permanent, so it can be removed when not low smoking. The cork in the vent hole is a good idea, because, when smoking, your vent should be opposite your coal pile.
I also put a nut thing in the drill hole and it seemed like it heated up differently than the metal lid, which caused the lid to warp at the drill hole/nut thing.