My bidon cage system had a minor improvement recently so I thought I should post the latest version info.

The change was to using clear tubing as insulating protection where the cage contacts the stem (I’d previously used various rubber pads but their attachment wasn’t ideal – no issues in practice but “could do with fixing”). The other components are two aluminum strips (8mm by 3mm), some fixings & two 58mm ID O-rings.
Taking a standard bidon cage & assembling my bits, it’s ready for attaching to a Brompton stem…
Once in place, the system looks like this…

Probably my longest running & most essential “hack”, the system performs perfectly. A bidon is held securely & the system doesn’t move around, although it’s also able to be swung left or right if necessary (e.g. folding the bike with a luggage bag in place or folding the bike for packing). The system is able to be removed readily but I’ve actually never needed to take it off. Of course, there’s bound to be another version at some stage – & I’m thinking 3D-printing could be utilised?