What might be cool is to make a flat panel like you describe but hinge it and put a vacuum motor or electric actuator on it to open up on fresh air settings....
All the old cars came this way, 50's and earlier anyway. The fresh air vent was typically recessed, with a flat cover that was sealed with a gasket around the edges and controlled by a lever under the dash. Kind of like the air grabber scoop on the roadrunners, just manual.