By making a mud flap? Vulcanized rubber would probably be the longest lasting material, or saddle leather (7oz.-12oz.) would work too. You could glue/cement/epoxy it to the inside edge the fender, or if you don't mind drilling holes you could bolt/rivet for a more serviceable option. Good luck on your endeavour!

I have in the past, told of using the tips of plastic dirt bike fenders as mud flaps ,they can be found cheaply on E bay ,are semi -flexable,can be painted (black would be good) and should provide long service,should it come in contact with the wheel or swingarm it should hurt neither.