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Depending on the size, I’ve needed a bit of extra support to clear a front Dynamo light. Do wonder if the WW/Swift etc bags have a bit of stifferner in the base to help keep their shape. Apparently correx board can be used as a stand in if needed.
Here’s my zipped roll plus support, not quite superC but might give you some ideas depending on clearance.
https://www.lfgss.com/comments/15500691/
As @yoshy said, can be a bit saggy!
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Here’s an example of a minimalist myog rack plus larger Carradice bag
https://bearbonesbikepacking.co.uk/phpBB3/viewtopic.php?f=13&t=19391
Plenty of people do it, they just do less loud posts about it on the internet cos it didn't cost them 3 figures and doesn't require validation