I have been using cut up old rubber boots for flaps since I was 14. I think that one is still on my dad's antique Rudge. My custom Roadster has them also. You can cut them so that the molded shape blends into the fender shape. I cut DIY stainless brackets for the fenders and put the flap inbetween. You can hang them farther down, since they are flexible for when the wheel rides off a high curb. If necessary I wrapped a stiff wire around the flap half way down to retain shape in the wind. The fenders are also likely 30 years old SS, I found at the used bike co-op. WAY better than the tinfoil VO ones I had for 2 years.