Thanks for the replies! Stopped by the local hardware store, not the big orange box!, and bought a black stair tread. Made up a pattern and cut them out. I used aluminum pop rivets with a backing washer to install them. I just made them from the front. Have enough material so maybe I should make rear ones also.
We just came back from a cross country tour and we are heading out on a short ride on the GAP with relatives who have never toured. Thought this would be a good chance to see how the flaps perform. Also the relatives!