I'd like to add a detachable shoulder strap to my waterproof panniers. I'm thinking of the kind of nylon shoulder strap with plastic clips that attach to a plastic loop or triangle sewn onto the bag. I've seen this kind of a mod on a pannier before, but I can't find any instructions or experiences of people who have done this sort of thing. I'm concerned about sewing something into a waterproof pannier. It probably won't be waterproof in that tiny spot anymore, but would that seriously compromise the bag and put my belongings at risk in the rain? Does anyone here have experience with modding waterproof panniers?