So my Lynx has two settings, One higher and closer to the infrared burner, and one lower and further away. I try to fit on the closer one but sometimes the wing or body might rub the the top. This one just barely made it. I have to remove the grill rods and the warmer rack. I place a large drip pan underneath so nothing drips into the grill. The drippings were used for the gravy. Supposedly my rig can handle up to 25lb but I think a 20lb bird is about the limit on diameter. I've never tried on my weber, don't have the set up either.
The infrared burner has only one setting, on. No adjustment for flame on this one, but not needed. Just make sure bird stays tied down and then wait for internal temp.