I believe 6/7 oz. should do it for that mail bag...also, since you are familiar with Tandy/Al Stohlmn books, look over the saddle bag patterns they have available. They are a bit smaller than we want our bags to be, but just take the sections and copy them up to the % you want, and wala, you have patterns. Just add dees on the sides, a strap and you're off!!