Out of a T-shirt, shoe laces and paint (:
Basically, i just like finding ways to waste paint on things.
I didn't take pictures, so please enjoy my wonderful drawings (:
Cut down the side of the tube and turn it so that the bottom edge of the shirt is now the top.
Decide whether you want to use the outside or the inside of the shirt as the outside of the bag; some shirts have a smooth side and a slightly rougher side- depends on your preference.
Lay it flat and get out your cassette and paint.
For the casing, you can do it the regular way and make the casing, then string the straps through, or you can just fold and pin the fabric down over the straps and then sew it. Either way you do it, make sure that you put the strings in right.
When you pull the strings, the bag should close. Don't mind how low i put the strings in the picture, that's just for reference :P