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Love to see it. Could you share a pic in use?
I'm not sure if this is what your after, but here you go..
.. I thought it might be a bit wobbly but it's actually been rock solid. I guess whole thing weighs about 450g which, considering I'd probably put a 300g mudguard on if it wasn't there anyway, doesn't seem too bad..
I've toyed with the idea of trying something like this (also on the rack) instead..
.. but I'd probably just end up cutting the straps off and ultimately losing some storage/flexibility so would just be a vanity purchase really.
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.