I would suggest a maverick et-732. To me it gives more options than a thermometer hard mounted to your smoker. That way you can monitor your temp at the smoker and from inside the house.
I just drilled a 5/16" hole near top of drum. That was just big enough to get the probes thru. I put cooker temp probe in the middle of the grate I'm cooking on.
You could get a hard mounted thermometer, but they can be up to 50 degrees off from the center of cooker. So you could get one that can be calibrated, and adjust so it gives you a reading of center of grate. But to know the difference, you'd need another thermometer to read temp at center grate.
So at that point I'd just get a maverick. I love mine! :)
UDS, mini wsm, 22.5" weber kettle