After a few trips with seat packs and their annoying swaying, we acquired some Tailfin racks. They are everything they are reputed to be! Zero movement and you can carry way more.
I got the aluminum one so I can mount stuff to the sides on the eyelets. Ms CE got the carbon one bc she's fancy. She also has the Tailfin Aeropack version, where the bag is integrated and very nice. I'm using a Rhinowalk 20L duffel from Ali.
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