I hate to plug NB since it's owned by AB now but they just happened to be the first link I found for a cooler bulk-head fitting. If you can find it somewhere else, go for it.
With the bulk-head in place you can grab any 1/2 inch NPT thermometer and screw it into the bulk-head. Should work great. Good luck, hope this helps!
Thanks for the replies. I should have mentioned that there's already a ball valve where the cooler's spigot would normally go. The thermometer would be in addition to that... somewhere on the cooler wall.
Yep, just find a bulkhead that has spacing for a cooler (coolers have a wider wall because they're insulated). Some of the ones out there are only spaced for stainless steel (very thin walls) and are very small.