I added a wood collar to my mini fridge, added insulation board to the inside. Also installed one of those amphibian heating mats to the fridge wall; this allows me to use the fridge to bring temp down & the heating mat to raise temp.
I attached my temp controller probe to the exterior of my carboy. I don't necessarily care about the exact "internal" temperature, I just want to have repeat-ability.. Let's say I made a really great IPA, and during fermentation I set the temp controller to 65 degrees. Long as I brew the same recipe again, follow the exact procedure, attach the probe to the carboy outside (as before) set my temp controller to 65 degrees the fermentation profile should match that of batch one..