In just did this the other day. I made a thermowell out of a chrome toilet supply line (there's a thread on here about it, very easy). I drilled a hole in my bucket lid with a dremel. I then got a rubbber grommet that fit really snug around the top of my thermowell. I put the grommet in my new hole in the lid and then slid the thermowell through the grommet. Works great. The grommet size I used is just slightly larger than the one that comes with the bucket. I don't know what size hole I drilled because I just free-handed it, but it was bigger than the bucket airlock hole by at least 1/8" in diameter.