If you like Domke bags, I'd go with the suggestion of the F-2, or they make a slightly larger version, as I recall, if you need a little more space, though I don't think you would. They're easy to work from, don't have loads of bulky padding, and don't slip off your shoulder so easily.
I'm also a fan of Think Tank Urban Disguise bags. What you have probably would fit a UD-50. Also easy to work out of and great for travel with well-positioned pockets and compartments.