A good seamstress can measure the haversack, and calculate the seam allowance, so you don’t have to take it apart. The straps should have their ends sandwiched between the body of the bag, and two medium weight leather pieces, glued, and then saddle stitched in place. As for a weatherproof coating, I would use sun thickened linseed oil from the art supply store, powdered pigment ( such as masonry cement tint, artists dry paint pigment, or even chalkline chalk) and some Japan drier. This should be left to dry indoors, in the fall, or winter, since the drying process can cause spontaneous combustion in warm weather, or when exposed to direct sunlight.
Hungry Horse