Gem Products 774 Stainless steel 4" round adjustable Gemlux butterfly vent ventilator
This vent for holes up to 3 1/4" in diameter.
For holes up to 4" search "vent 775" for a slightly larger 5" version.
These vents are 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.