The Sherpa rack looks pretty impressive, hadn't seen that before. Steel, and mounts to the seatpost with two arms spaced somewhat apart, so it should support quite a load.

I note that they offer shims to adapt to seatposts anywhere from 27mm to 31.6mm, but I don't see anything saying what size it fits without the shims -- will it fit the Swift's 34mm seatpost?

You might also consider Carradice's SQR (Seatpost Quick Release) series of bags; I've got the 5 liter Glentress, obviously too small for your needs, but the Trax, Tour, and Slim are all 16 liters (they also have a mount to which you can attach your own bag), and the system is rated for 10kg (22 pounds) capacity; most of the other quick-release bags I considered were limited to much smaller loads. Might work well in combination with a smaller front bag or rack (say 5lbs front, 20lbs rear). The advantage is, when the bag is dismounted from the bike, the only bit remaining/protruding is a small (heavy-duty) plastic block on the seatpost. I posted more info on the Swift folders thread here.