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I have a Weber thermometer in the cover of my grill and it works just fine. You can pick up one at any Home Depot or Lowe's..........or you can call Weger direct. They will be glad to send you one.
I have a bag of corks I use in the vent hole, (1st drill a small hole for the stem) either for probes or I use my weber gas thermometer because it has a long stem.