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Another option, that requires a larger hole is to go to ereplacementparts.com and order the Weber probe grommet.
It's the silicone grommet that comes on the newer Weber Smokey Mountain smokers. Requires a 1 1/4" hole to be drilled and the metal can't be thicker than 1/8".
I have used the "Cable Entrance Connector" available at Lowe's for a couple of bucks in the Electrical dept. I think it uses a 7/8" hole and has a rubber grommet in it.