If you're using it to ferment, I'm assuming that you have an airlock or blow-off tube coming off of one of the posts. If that's the case, you can always unscrew the pressure relief valve and see if you can find (or make) a thermowell that would mate with those threads. That way it would be centrally located in the keg and would be easily replaced with the pressure relief valve if you need to use the keg for pressurized kegging duty.
(Edit: Not being familiar with 10 gallon cornies, I am assuming they have lids similar to the 5 gallon versions with the pressure relief valves. If not, ignore this suggestion.)