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I have a Camper with the larger quick release bagman. I think because I use the qr rather than Brooks saddle loops, for e.g., the distance between the attachment points and the 90* bend at the bottom of the bagman is smaller and it squishes the bag quite a lot. I know the Camper is the biggest they sell, but I'd say good shout with the larger bagman if you are going to use the qr functionality.
I put a camper longflap on a sport bagman a few times, it didn't look totally weird, but it lost some volume obv.
How about a lowsadde longflap with a sport bagman?