If you're going big, go Spike Brewing Custom and have them add a spare 1.5" Tri-Clamp (aka Tri-Clover) for a thermowell. Somewhere around 6" from the bottom near back side. Same height as the analog on the front.
Then grab a Tri-Clover thermowell at Brewers Hardware. The 6" works well on the 50 gallon kettle.