I think this may appeal to several of you--i made it myself, and anyone with rudimentary sewing skills could do the same. It'd be a great first project for those wanting to learn as well, because it doesn't involve any complicated construction, just a few straight seams.
I drafted my pattern working up measurements from existing bags of the size i wanted, but the big pattern companies (Butterick/McCalls/Simplicity) do accessory patterns and one or more of them probably have a courier bag pattern for purchase, if math and drafting is not your forte. If they aren't doing a courier pattern right now (since patterns run in and out of production according to style), you can easily mod a diaper bag or tote pattern into a courier style--eliminate the baby-centric bottle-pockets and such for a diaper bag pattern, or add a flap and move the strap attachment location on a tote pattern.
The bag is made from one layer of black brocade and one layer of heavy denim for durability. I cut out the pieces in both and treated them as one layer when sewing. It's also lined in a plain black liner fabric.
I did a section of the flap in a plain black satin instead of the brocade, because i wanted a flat surface for the iron-on, which is an image of an old tobacco promotion coupon. You can turn anything into an iron-on if you have a decent printer and a pack of transfer paper (which can be purchased at office suppliers). The strap is cotton webbing, to which i added shoulder padding from industrial felt and a layer of velvet:
You can also see how many times i've repaired this bag with patches where the brocade has worn away--if i were building a second one (which i'm sure i'll wind up having to do within the next year or so) i'd build the back from leather to avoid this kind of friction damage where it rubs up against the body when the bag is carried/used.