So I didn't want to drill a hole in my tun and worry about sealing it and all the jazz.
My remedy was a cheap analog thermometer
and the cap from one of the new Tropicana OJ bottles.
I drilled a hole in the cap and pushed the thermometer through. Then I just set it in my mash once everything is stirred up and good and leave it in the tun the whole time. It fogs up a little but it floats and it lets me know instantly if I lose any temperature.
When I mash-in I have a $20 digital thermometer that is waterproof that I dip in and swish around to make sure I hit my mash temperature. Then I drop the analog in the tun with the OJ cap-floatie and close the lid. I check it every 20 minutes or so to make sure its good.
Simple fix, $30 tops. If I ever win the lottery I may but a Thermapen but that overpriced accuracy can wait. For all my homebrew purposes, my set up works just dandy.