I have a old smokey BBQ thermometer with a 2 inch probe. I have heard different stories from different people. What's better mounting right under cooking grate or on the lid?
I would mount it right under the grate. That is where the temp is most important. It has also been my experience that the UDS without a diffuser is about 75 degrees hotter in the center of the grate than near the side. This is usually no big deal but you do have to keep it in mind and adjust cooking accordingly.