A bit of DIY. get a couple D rings some small P clips to go around the straight part of the D, and some sheet metal screws with a round head..

screw thru the canvas into the end of the wooden dowel supporting the Carradice bag , and you have someplace to clip a strap onto ..


Alternatively Rixen & Kaul -Klick-Fix makes a number of bags that use rear facing seat post mounts they also make
including an actual backpack ,,

and theres the Carradice SQR http://www.carradice.co.uk/index.php...product_id=138

http://www.carradice.co.uk/index.php...&product_id=57