Well, I had a good look at the real thing, and worked out the basic shape and design (which is very simple for the sling bag, it's basically a flat square, with the corners flattened across to make a basic bag shape). I made the lining first (out of cheap ikea canvas) and needed a couple of tries to get the shape right, but that was OK as it was cheap fabric - then when happy I made the pvc outer to the same shape, and fitted together. I added external pockets as well which OK bags don't normally have. I used a long strap from an old worn-out nappy bag.
I am crafty anyway and have a lot of experience with sewing - it was quite hard to do, probably not ideal for a novice. but v. satisfying.
(Nonetheless I still yearn for a real one!)