You should be able to get the bag frame , from SJS in UK it's listed,
if not thru your dealer.

a sleeve on the back of a messenger style bag pattern, with a couple velcro strips across the bottom opening would work.

thats common to most bags made for the rack, I think the O bag integrates the mount in the bag, but as
it is almost $300, I havent seen one, and its only the volume of 1 Ortlieb front roller pannier.

the 2 pockets on the back of the bag are handy, I have Water bottle in one , puncture fix tools in the other,
a battery head light , stows in there too..