Weber modification

I put a Tel-Tru on my dad's Platinum.
Measured from each side of the handle bracket to the vent rivet and then marked that off on the opposite side.
Drilled and mounted it there. Looks good.

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?

This location works for me. I like to cook indirectly with coals on opposite side from temp gauge. The food is under the vent.
All my kettles have one in same place.

Mine are all mounted about 2-3 inches above the grill grate on the side. I use Char-Broil thermometers I got at Home Depot for about $8. They are pretty accurate within 15 degress or so.

Buy the therm from the performer and install it yourself.

I think this is a hard one to accomplish, as the style of cookings you can do on a kettle can have varying temperatures at varying locations. If it is for indirect cooking, I would be using the thermometer to ensure I do not go too hot, so I would place it near the exhaust vents, as this should be the hottest place with the exception of right above the coals, but you aren't cooking over there. This would be the best location if I was to install a thermometer. Direct grilling could probably be satisfied with a thermometer anywhere in the dome so once again, near the exhaust vent should be fine. You actually cooking temperature will probably be lower than this number though.

I just stick a turkey thermometer through the vent hole.

For my 22 1/2" OTG, I bought a replacement Performer thermometer, and put it in the same place as it is in on a Performer.