On my most recent tour I carried two Walmart duffels. As it turned out I only needed one, as it was big enough to hold all of my gear + panniers (except handlebar bag which I carried separately with valuables), yet still met the airline's requirements for checked luggage size. When on the bike they fold up quite small and fit nicely at the bottom of one of the panniers. If you go that route, get the lightest and most basic one you can find. I didn't want any wheels or handles that would make it heavier or more bulky - just a light bag with a shoulder strap. I don't know what material it's made of, maybe polyester. I think they were ten bucks at Walmart.