Travelon's Pack-Flat Back-Up Bag
is designed to stash nearly flat in a wheeled carry-on bag. Measuring 19 inches by 13 inches, the bag unzips to become a spacious 10-inch-deep carry-on-size bag.
Made of heavy-duty nylon, the bag can be carried by the double-
sewn handles or adjustable shoulder strap.
I would have liked double zippers so the bag could be padlocked. I also wish the shoulder strap were detachable. All in all, though, this is a great extra bag
Pack-Flat Back-Up Bag (22825) is $24.49 at