Another vote for the Ortlieb bag. I've taken one of those mounts apart and honestly even though it seems complicated the mount is actually one of the best designs I have ever seen. There is pracically no possibility where your mount can break and the bag can just fall off while you are riding, unlike with some plastic mounts. The cable is more than a fastening system. It's also a very good security system. The thing is also really difficult to rip off or cut off for a prospective thief, not to mention you can lock the bag so it's locked on the mount and locked shut. Then again the locking system only works for runners and it won't protect for a second if you are not on, or directly next to the bike (nor should it really...)