matt,
have a look at Carradice, they make cycle bags out of Cotton Duck (fibres expand when they get wet making it waterproof, therefore no 'waterproofing treatment' to wear out). I'm not sure where you get the material, but I've been using their bags for a while and they are tough; much tougher than Ortlieb.
a couple'o longflaps on your BSA would be 'period' too, once they'd worn in, as I believe the designs haven't changed in donkeys years.
cheers
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