For BROMPTON seatposts
These saddlebag mounts from Frost & Sekers in London are the perfect solution for those of us whose preferred saddles don’t have bag loops. The system includes a mount that gets clamped in with your saddle rails (longer bolts are supplied in the package), and a handle that mounts to the bag. To attach the bag, you just slot the handle into the mount until it clicks. To get it out, all you do is grab the handle and pull (You still do probably want to use the seatpost strap in the normal way though).
What’s particularly nice about these is that the overall weight is light enough that it isn’t such a big deal to just ride around with the mount attached when you aren’t using it; and unlike other saddlebag mounting systems, leaving it in place doesn’t get in the way of attaching other styles of saddlebag or basic seat packs. They’re even compatible with the Dill Pickle tool canister!
The Brompton-specific version adapts the Brompton seatpost clamp. And the handle is a great way to wheel the bike around when it's folded!