Here is one mounted on an 18.5 OTG. Similar to what Harbormaster suggested< i placed this at approximately the same distance from the handle as the vent. I used a Q-grill thermometer that I got off eBay. Put SS washers on both sides.
BTW - note the reading. Who says you can't get a kettle to reach decent searing temps?
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, KJ Classic, and Schwenkgrill