I bought a thermowell from this guy.http://www.stirplates.com/tWellStraight.htm
Take a standard carboy cap, cut the narrow (long) tip down to 1/2". Boil to soften and run the T'well down through it. Attach a 5/8" id blowoff hose to the center tip and you're good to go!
FYI a fan in the fermentation cooler will speed heat transfer and prevent undershooting your temps. If you are using a 14-15cuft freezer and a 5-6 gallon batch, just put a carboy full of water in the cooler too, that will add enough mass to absorb the undershoot of the air temp.
I have found, however that an insulated probe taped to the side works just as well, but for beer's sakes do one or the other!