My Brompton with a backpack on a C frame (frame is cut back so it is narrower than regular C frame). Think it is a Hagløf XL backpack. Mounted across the frame, top of bag facing the camera.
https://www.flickr.com/photos/778361...posted-public/
My sons B with a "cabin luggage" bag atatched to a regular frame. Good thing about this solution is that it can be rolled with frame still atatched so no need to carry the lugage when not on the bike.
Both lugage options was atatched with two of those straps you use to make sure the suitcase do not open when in use.
https://www.flickr.com/photos/778361...posted-public/