A Carradice bag with their Bagman rack, quick release version, would work great. You can easily attach a shoulder strap to any Carradice bag. They have metal rings in the top corners for that purpose. I've used a Carradice Barley as my main commuting bag for 7+ years, just recently switching to their Pendle bag, which is the next size larger. If you order direct from Carradice in England, they have much better selection than US dealers and lower prices. Even with shipping, you save a lot. Remember to subtract the VAT when calculating prices; I think that reduces the price by about 15% if I recall correctly.