Oh wow. That's a beautiful backpack. I'm going with 20 oz khaki duck canvas, and a similar modular idea. First I'm making the main bag, with a drawstring neck, and a flap top, secured by a pair of leather straps/buckles recycled from some old belts. I took the measurements for the straps from my current backpack that fits really well, and I'm trying to find a good system to make the bottom part of the straps that attaches to the bottom of the bag adjustable, and I want to do it out of leather and buckles like the strapping if I can. Before I sew the main 'box' of the bag shut I'm going put double layered reinforcing straps of the canvas along the inside of the panel that faces my back to help anchor the area where the straps, flap and top handle of the backpack all come together. http://imgur.com/a/M3ZwX Here's the first tracing and cut of the bag, I modeled it out of construction paper first, and I'm sewing it tonight, couldn't do it last night because I need a denim needle for the sewing machine I'm borrowing.