Originally Posted by MichaelW

You can add a Carradice Saddlebag; if your saddle lacks the rear slots you can get a set that bolt onto the saddle rails or an "uplift" frame which bolts to the seatpost.
Some of the carradice models use an SQR quick-release system.
Saddlebags can take as much volume as a backpack and a much heavier load. They dont need the big, heavy metalwork of racktop bags so have the best weight/volume ratio of any luggage system and are very aerodynamic.