I also use the 12" turkey fryer thermos. I actually have 2 in mine, one about 1/2" under each cooking grate. They're cheap, easily replaced and I've found them pretty accurate.
Big JT's Smokin' BBQ Competition Team
“If we could change ourselves, the tendencies in the world would also change. As a man changes his own nature, so does the attitude of the world change towards him.
We need not wait to see what others do.” - Mahatma Gandhi