Jill from Creating My Way To Success made this messenger bag from an old pair of jeans and some cute cat fabric. She shares a tutorial showing how she did it. She included a zipper across the top (under the flap) to keep the contents from spilling out. The jeans waistband becomes the strap, and the back pockets will become pockets for the inside of the bag. Get the tutorial at Creating My Way To Success.
[photo from Creating My Way To Success]