I just came back from a trip with the Brompton. I tried to finish the DIY luggage system before going on the trip, but made a rather bad mistake and would have had to re-do the piece that mates with the mounting "triangle".

Anyway... during my trip I found a shop that stocked the Brompton front luggage system, and got the triangle thing and the bare-bones carrier without a bag. I'm using it with bags I already have (messenger bag, backpack, etc.) and it works pretty well.