Ideally, I would suggest using a thermowell. The guy at Stirplates.com ( a vendor here also) does a great job and good prices.
On mine, I share the t-well and a 1/2 blowoff tube in a #10 (? ) stopper on my better bottle. As a test, I set my freezer to 66*, waited 12 hours, and measured the temp inside the thermowell. 6* difference! So, when I thought my beer was fermenting at 66, it would have been 72. Fixed that !