This is my idea regarding the mechanism, it is a simple lever. When the lever is in vertical position the vent is completely air/water tight. Also by taking a glance at the lever you can determine from a distance that if the lever is vertically positioned then you can be assured that the vent is closed. My only concern is that the lever may be a little bit in the way but can be shortened. Also to compensate for this I am going to upload a design with the same mechanical principle but with a rotating handle.