Really wanted to get away from Vacuum operated scuttle vent and found that some models had a lever under the dash so...
after paint, i installed the new rubber gasket under the vent and fabricated a spring locking mechanism so that in either open or closed position, there is tension keeping that position.
first photo is original equip.
second photo is my McGyver version...i had to pull back the cork gasket on the bottom of the flap, clamp a piece of 1/4 inch aluminum plate on the back side to prevent burning through and welded a piece of sheet metal to the flap, because this model vent box did not come with the slotted metal in photo # 3