Well, that's why I asked about prototyping. It's great to be able to plan physical objects on paper, but for something that you're making custom for yourself, it might be worth making a prototype to get an idea for fit, weight, etc. That is a nice looking backpack though.
Should I make it out of construction paper or something to get a better idea of dimensions? Also if you feel like dropping the cash this is the website of the guy who makes them.
I think construction paper might be a good idea, as you can figure out how much stuff you'll typically carry and how to distribute it. Since you have that website (thanks for the link by the way) you might even ask the guy if he could give you some tips, or if you can bounce ideas off of him. Incidentally, last night I spoke to a good friend interested in opening up a food truck or a lunch counter. He met a guy involved in that stuff, whose kid happened to go to the same preschool as my friend's kid. They ended up hanging out and through talking about my friend's ideas, the guy ended up hiring him to manage a cafe. Your situation is a bit different, but there may just be some interesting opportunities for your interests to work to your advantage. Just my 2 cents.