Well it's a little embarrassing, but I did a super-budget temp gauge on my rig- I got one of
these, and used electrical tape to attach it to my mirror pointing front to back. Easily read through the drivers window, and works as a conversation piece too. For maintenance, I simply take it off and calibrate it via the ice-water / boiling water method maybe once or twice per year. Its a budget mod (if you can even call it that) and it has worked well for many years. As an added bonus, you can use it for coking at camp!