I have tried a cardboard box, strapping the panniers together, and a large duffel.
- The duffel worked best if you have a place to leave it, are willing to pitch it, or are willing to ship it home.
- Cardboard boxes worked OK, but the airlines made us sign something to say that they were not responsible for damage. They also stamped that all over the boxes.
- Strapping my panniers together worked OK on the train, but I used them as carry on. You would have to do a much better job of strapping them together before I would trust them to baggage handlers.