I use this fitting from McMaster its 1/2 NPT male pipe thread on the fitting side and about 1/2 diameter rubber gasket on the probe end, they are designed for waterproof cable feed through applications. Just hand tight on the hex nut is all thats required for a water tight seal, probe shown is a pinpoint. I will look up the part number if your interested, they only cost a couple of bucks also probe is easy to remove for cleaning or replacement.
Jim