I just got an Carradice SQR Tour for a similar purpose. It's a bit on the big side but is not too heavy and it doesn't get in the way at all.

The mount is rock solid with no sway at all. It also acts as a rear mudguard on bikes without fenders.

You will need a non-carbon seatpost to mount and enough seatpost exposed for the mount to attach. It's an amazing bag. I could post a photo or two with my Specialized Roubaix if you like.