One thing I insist on for any flap I will put on (especially in front) is that it crush without damage when I wheel the bike off the sidewalk (often with loaded panniers). And it better crush, not destroy the fender.) Haven't gone to leather and I like the conveyor belt idea. What I've used with success is thin architect's film. I cut two layers, tape them together with good packing tape around the edges. Stiff enough to stay in place for deep puddles but crushes easily and with very little damage when I drop the loaded bike on them. Sadly, my stash (rescued from the trash of the neighboring architectural firm 20+ years ago) is running out. Folks here in Portland are fond of using cut down waterbottles for flaps but I wonder if my fenders could handle my "drops" with a flap that stiff.