I have made them from leather scraps. I have some waterproof black leather, presumably chrome tanned, that is good for this purpose. I have also used a veg-tanned horse hide that might be a little too flimsy but it has worked so far. I mounted a red reflector on the rear one, which keeps it from flopping around too much. The front one is pretty wide, and I thought it would take a scoop-like shape, but when it's dry it curls up to the rear. When it gets wet, it straightens out and actually drags on the ground. That isn't a problem, though.

The end of the fender has to be lower than the axle; otherwise the mud flap rubs on the tire and can get pulled up under the fender. That has happened to me, and it's definitely a problem.