So not only is Dave a good musician, he's also excellent at modelling messenger bags. What talent.
This is the bag I made for my brother, finished just in the nick of time. It's based on Simplicity 9000, like the one from last week, but I made a few more modifications. Instead of sewing it all together with bias binding, I sewed all the pieces to each other. I also chopped apart the pattern piece for the flap, added seam allowances, sewed the top and bottom flap pieces together and turned it inside out. Then I put it all together kind of like I put a tote bag together (see this Totes 101 tutorial), being sure that I pinned the strap and the flap to the right seams before sewing.
This beastie came from my sketchbook, via the power of the Transfer Tool.