AGU bags, made in the Netherlands, are weatherproof and very good values in the US right now due to the exchange rate. I bought an AGU Qyoto handlebar bag from Nashbar last fall for only $20.
Very good overall, except for the zippered side pockets (the zippers work pretty much like ziplock slider baggies, fussy and don't always secure properly), which weren't a priority for me -- I was mainly interested in the main compartment, which matches a favorite camera bag and even accepts the same padded inserts. Outstanding value since the KlickFix adapter alone (included with my bag) sells for $20.
But with the KlickFix mount you don't really need the porteur rack, although I see no reason why you couldn't just use a cargo net to secure the bag to the rack.
Nashbar doesn't currently stock AGU bags, but I see a few European retailers making it as easy as possible to sell to US customers, offering readily accessible dollar figures, free or discounted shipping, etc.
Other than that, I'd look for any weatherproof rolltop bag. It's a traditional design used by some custom leather and waterproofed canvas bag makers as well for travelers who like the design and look. Nashbar's own brand looks like a good value.
Main reason I preferred the AGU handlebar bag was for the easy access. It's secure too. The canteen shaped bag lid folds away from the cyclist while still on the bike and needs only two bungee cord type elastic closures over rigid pegs. Quick and easy. There's also an inner ditty bag incorporated with gathered top secured with a drawstring for added security. I've ridden some rough roads, washboard gravel and even fallen once and nothing has popped out of the bag when the lid is properly closed.
It's a plasticky feeling bag, with a weatherproofed ballistic nylon outer shell over a rigid plastic enclosure -- not sure whether all AGU bags follow this design and build. But I expected it to feel plasticky since it's intended to be weatherproof. The gray main material is offset with tan leather or leatherette hinge and tabs.