Originally Posted by Paulash

I would drill a 3.5mm hole right through the tube for the long bolt and secure it to the opposite side of the tube with an appropriate nut, washer and Loctite. 3.5mm is a small hole but could it weaken the frame in any way?



This is my back-up plan if the homemade HXB doesn't work out.

Thanks again to you all for your help. I'll keep you posted.

UPDATE: Brompton's Zendesk recommends against drilling into the tube. So, a Shimano derailleur is the next move.