In the wind tunnel videos I saw, on the F150 it completely disrupts the airflow against the side of the truck about 8" high, reaching about 16" high by the back of the truck. I think I could stick a lego house there and it would be better.
That said, in most UCC states, the requirement is to be able to see 200 feet behind the vehicle, and a little math for my vehicle tells me I need a 4.5" extension to meet that requirement. For shorter vehicles, the distance will be less. Obviously, if towing, on a truck the mirrors need to be wider.
Design issues!
The second idea I have is a small pyramid inverted on a long or extendable stalk. Like the 2nd attachment above.
The third idea I have is for a slatted mirror that has many vertical elements. It would allow air to pass through the slats, but from the driver's view, the mirrors would be overlapping prisms and provide a single image. However, this would be a PITA to make or adjust.