If you are invested in Brompton Bags, you might want to use them with your other bikes, too. Many Riese & Müller E-Bikes, such as my Charger 4, come with four M6 threads at the side to mount either a bottle holder, a foldable lock, or one of their front racks. Why not use them for a custom adapter?
I got some stainless-steel parts from a local metal shop and screwed them together. I choose mostly from standard parts like rectangle tubes, L-profiles and sheets and let the metal shop cut them for me. Instead of an original Brompton front carrier block, I used 3rd-party one. If you choose the original part, I would strongly recommend to flatten it on the back side.
I will look for shorter screws and paint it black, but I’m still convinced, that this adapter will be used a lot. Feel free to rebuild it for your bike or adapt it to ISY or QiO bike, which come with similar threads.