I put mine in the lid which was not a good idea. Mount yours in the mid section like others I've seen.
The ones I looked at had a 2 1/2" probe. That should be sufficient.
I have an ET-73 and ET-732 that I use when I smoke. When I was using those on a bigger smoker, the lid thermometer was good enough. I had enough experience using it in tandem with one of the other thermometers that I could work with the readings.