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.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team