I have seen.. from the grills I have had here in Europe .. the older grills have tighter fitting lids. I have one '96 18" that fits so tight that with out the grates the lid will pick the whole grill off of the ground. No way in hell you would ever get a probe in between it.
New grills .. that's another story. There is so much play in the lids that you could fly a Boeing 747 in the gap.
For this reason I never clean the lip or sealing edge on a new kettle.
As far as cutting a slot in a grill.. I would not do it to the grills I have only because I don't want to risk rust or damage.
If I had a cheap used dedicated smoker then maybe.. but I don't have it now.
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