89Hilo,
You can get some foam at a craft store and cut it a little larger than the screen opening. Push it in the opening and it will stay up due to the "tension fit". They do sell small "silver quilted" covers for the vents. They are the same type as the automotive windshield shades you see in parking lots. I believe this kind of sun shade is attached to the vent by Velcro strips.
Bob