If you have a Weber kettle, use THAT lid.
If you set your grills at 2 inches below, and 12 inches below the top,
youi just got another cooking surface.
If you use a flat lid, you can't do the 2 inch measurement,
and also can't use the lower grate.
I've been using a Weber kettle lid, for 3 years now,
and would NOT go back. Ever!