You should already have bought a bag support frame.. from Brompton.
For the snap together connection with the frame block , you know..

I have a messenger bag that came with a waist belt , and the shoulder strap.

wrapping that waist belt around the tubes of the bag support frame ,
and clipping the buckle shut is all I did to combine mine..
that and a stiffener of coroplast from the last years political lawn signs.
stuffed in the bag.

But really you can use their Touring Bag as a messenger or shoulder bag,
off the bike. its bigger..
with most of their bags there is a frame pocket on the back,
opening just big enough for the handle on top.
velcro straps close the bottom to keep the frame in side the pocket.