I wonder about the contrast with the Brompton T-Bag that, at 30l capacity, is nearly 50% bulkier, yet sits rather firmly on the bike when fully loaded. What is the weakness in Freepack Meta that makes it unsteady? Could it be fixed?
It is possibly not the bag itself. The Brompton-carrier-block is bolted directly to the frame whereas the Klickfix on ijsbrand's photo it fitted to the Tern luggage truss. The truss creates a lever and thus vibrations etc. have a bigger effect. I noticed the same thing when having load on the kanga-rack fitted to the luggage truss on my MK X. I used the kanga-rack on a Brompton as well (via a Klick-Fix-to-Brompton-adapter) and did not notice this effect. Therefore it seems to be indeed the luggage truss that causes vibraton. It is probably made for light load (and possibly for small bags, that do not virbrate within your field of view).