If you really want to get technical about it get a stopper with 2 holes. Take a piece of 1/4" copper or stainless tubing(I used the diptube from an old corny keg) and seal one end. I smashed the end of the tube with a 2 pound hammer, folded it, and repeated twice to be sure it is completely sealed. Cut it so that it goes into the middle of the fermenter and stick the temp probe inside.
I have noticed that there's about a 3 degree difference between the internal temp and the strip thermometer on the side of the thermometer, more on my plastic buckets.