Hello everyone!
For those who own a Brompton & a 3D printer, you will find here https://www.printables.com/model/1820520-brompton-seatpost-backpack-holder my custom solution for holding a Trekking backpack.
I have been using it for 3 years across France, Belgium, Germany, and the Netherlands, covering about 3000 km on both SWB (1998) and LWB (2006) Bromptons. It works great and I thought you might like it.
by ARCH_KLM