Ultegra 8000 was just barely too big. Im going to try the Dura Ace 9000, Tiagra 4700 and the litepro mount with sram washer
Some combo of the above will work as the derailleurs are designed with that skinny arm going up and not bulky wide like the other ones.

It is no expected to be a press Enter operation

Here are 4x of the thinnest design derailleurs and less bulky for folding, i could find.
If anyone has installed them, please comment with some feedback, so I can save the time/$$ on some of them

- Ultegra 6700
- Ultegra 6800
- Tiagra 4700
- Dura Ace 9000

Looks like the last two (4700 or 9000) might be the best. The little arm design is pulled back the most, away from seat tube.
Winston member recommended Tiagra above, and 9000 used by the Brompton guy in his LeJog trip

Pics below