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In the end I resorted to the dry bag/Voile thing and used a cut up Ass Saver as a base like this (the saddle rail bit clips to the rack struts)..
Excellent, I did exactly the same with an IKEA flexible cutting board and some zip ties, total cost under £1.
https://www.ikea.com/gb/en/p/finfoerdela-bendable-chopping-board-green-bright-green-40559680/
Can't find any pictures but you can see a tiny bit of turquoise under the front of the drybag, shown also with ROK straps.
I've been waiting years for someone to come out with a minimal rack top bag that would be thin enough to work with a Tubus Airy (admittedly a niche market!). In the end I resorted to the dry bag/Voile thing and used a cut up Ass Saver as a base like this (the saddle rail bit clips to the rack struts)..
.. the cut outs mean I can also use it with (front) panniers, but mainly just with Voile rack straps and a cheap dry bag. The rest of the time it just gets left there as a shitty mudguard. Amazingly Gorilla tape has lasted >5 years.
The closest I have seen a 'proper' bag for my purposes would be one of the bigger fork leg or down tube bags (Apidura, Salsa or Revelate), some of which have a thicker base to handle the rub, plus their own straps. None are ideal though, and probably too small for most purposes.