I don't know how determined you are to building your own bacg, but there's a company out in San Francisco called Mission Workshop that makes absolutely stellar handmade bag that truly last. I've owned one for over a year and with daily wear and camping trips, it literally looks just like the day I bought it. I'm pretty certain these are the same guys that designed the fantastic Chrome bags as well. Anyway, they have a pretty cool system there where they make modular bags called Arkiv that are completely built out of what you choose for it, like number of pockets, size, additional things on top of the bag, but they're really fucking expensive if you take that route, so who knows if that's up your alley. Even if you don't want to buy from them though, they're a small company of complete enthusiasts who are stoked about what they do, so if you email them for advice on building your own and what materials to use, I'm sure they'd love to respond and help out.
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.