There are lots of travel cases which I think could be made to fit between the rails, or even a small bit wider. The advantage would be the length of the bag being able to fit much more into it and the strength of the rails being able to support that extra size and weight. It would be possible to use more than one bag if the user wished. Water bottles could be mounted forward of the bag too, maybe. So long as they were outside the pedal stroke of the rider.