Best stem length for mini WSM thermometer?

Well, I think it will depend a little on the design you are building. Are you going to have any sort of heat sink or diffuser?

I plan to go with something cheap and easy and then I can "calibrate" it and learn what the actual temps are and go from there.

I would try to get something that would read the temp. in the CENTER of the pot. Ex. if the diameter of the pot is 14 ", i would go for a 6" or 8" stem thermometer. This would put you 1" from actual center. Never seen a 7" long stem thermo. If you are going to use a short stem one, lets say 1 1/2" to 3", the temp reading can be off from 30* to 80* from the center. Unless a defuser will be used. then the differance from the outside to center will be much less.

Why do you want to permanently mount a thermo when you can drill a hole and use a probe thermo? You can place it wherever you want, it'll be more accurate and you can use it on other grills or smokers. Everyone keeps talking about how horribly inaccurate and unreliable the grill thermos are, but everyone insists on sticking them in their wsm's. I don't follow the logic. I've been thrilled with the way my ET-732 works in my mini.