Daniel: Have you considered the Carradice SQR system and, in particular, the Super C SQR Slim (Pat. Ped., TM, MR, all rights reserved, etc etc etc)?
The SQR system is a QR mounting bracket that clamps to the seatpost and holds the bag considerably higher than it would be if simply strapped to saddlebag loops, plus a frame to which the saddlebag straps and that slips into grooves in the mounting bracket, for quick on and off operation. The "Super C SQR Slim" adds to this mounting system a 16 litre saddlebag designed to minimize the distance required between saddle and tire.
My Matthews custom has a frame that, in Road terms is the equivalent of about a 58 cm c-c frame, but the 700C X 60 mm wheels are almost 30" tall, and anything besides a tiny wedge strapped to the saddle will drag on the tire. With the SQR system I can easily accommodate a Junior and even a Nelson if I am careful about loading; but the Super C SQR Slim gives me more than Nelson capacity and no danger of tire contact at all.
For comparison, the Saddlesack Medium, sans bulges, carries 18.6 litres. The Super C etc has a volume between that of the Nelson LF and that of the Camper LF -- pretty big.
Super C blah blah blah as mounted on my Matthews.