I would avoid going the leather and canvas backpack route, they weigh significantly more which could make the wear on the jacket much worse. I would find a high quality textile backpack that is as light as possible and modify the straps instead. The best approach would be stitching something soft onto the backside of the strap, or getting/building a wrap to cover the straps instead. You could also just stick some adhesive backed moleskin onto the straps which would be the easiest route. I do a lot of leatherwork and have made a few canvas and leather bags--the increased weight is remarkable and can be a deterrant for every day bag use. If your goal is to reduce wear on the shoulders of your jacket I would look at reducing weight over changing materials.