Gem Products 774 Stainless steel 4" round adjustable Gemlux butterfly vent ventilator
For holes up to 3 1/4" in diameter. Search "vent 775" for a slightly larger version.
This vent is adjustable using sliding knob. Commonly used on boats, RV's, and automotive applications. Several have been sold for BBQ's and smokers.
Mounting screws not included.