Promised for the photos and update
Two straps are here today quickly testing them
Mounted two straps at the bottom to hold the bag in place. Since the plastic bottom of the frame has taken the load, both straps are holding the bag toward the frame rather than taking the total weight of the bag.
Same for the buckles which hold the bag in place.
The top Velcro holds the bag to the brompton handle bar
Finished! Just had a ride seems this is very stable and bag doesn't shift at all.
Very happy about the results. Now I can carry the heavy battery at the back when pushing the folded brompton !
Pat