@DaGoomba
I second the river country thermos, I've been using them for 12 or 13 years on everything from stick burners to electric smokers and kettles. They are inexpensive, hermetically sealed, accurate and adjustable.
I just received a 5" I plan on mounting on my new 26" premium kettle. I know it seems like sacrilege to drill into new kettles, but mine are all intended to be users and a well placed thermo works wonders on achieving great, consistent bbq.