craftgrrl | Trying to make a sling backpack.

Years ago, I had a really cool sling backpack, but like the ~~idiot~~ person that I am, I threw it away when the zipper came off. I didn't realize at the time how rare it was (the stinkin' strap connects on the "wrong" side). Now, I'm thinking about trying to recreate it, but I'm not sure exactly how to go about it. I've got a whole bunch of questions that I'll try to break down into sections.

Design: The basic look of the bag is something like this or this. The biggest difference is that I want to reverse it so the strap connects on the other side of the bag (which means that the bag would sit on the left shoulder instead of the right). It also needs to big enough inside to fit a 1" binder.

I finally managed to find a bag with the strap on the right side, but it's too small at the top to hold a binder. I've been trying to create a pattern based in it, but without taking it apart, I don't know what the seams look like.

Could someone please explain how to create a pattern based on measurements from a finished bag (if it's even possible)?

Materials: My first thought was to try to use duct tape. I'm just not sure how durable that would be. I could try to reinforce the bottom with something (plastic?). The main advantage is that this shouldn't need to be sewed, but if it falls apart it's pointless. It also shouldn't require much in the way of seams (which means my pattern would work unmodified).

Beyond that, I really don't know what material I should use. My only comment is that I would prefer something less cloth-like" in favor of something more like a tent (nylon?).

Is duct tape a sturdy enough material to be used for making a backpack for daily use?
If not, what would be a suitable material?

For all I know, this is going to end up being way beyond my skill (but I'd prefer to have something to work towards to making a bunch of simple things I don't need). If there's anything else I need to know, feel free to say. Sorry for being so long winded.