I tripped upon this wonderful Dahon trunk bag at a delightful price. I had sold them for $69 a couple of years ago and now I offer you the same bag for $49. It has lots of room, plenty of pockets, and a great attachment system. Notice the bottom of the bag in the photo. That"s a base that connects to the rack on your bike. The bag bottom slides into the base and then clicks in place with a little tab.
Most trunk bags connect with velcro straps that weaken with time and with age many bags lose shape. This bag shouldn't have those issues. The connection system should help this bag give years of service. At this price you might like to buy serveral and give them to your cycling friends as holiday gifts.