I use the Bach Cargo Bag Lite to protect my trekking backpack on (air) travel. This task is performed the backpack to my complete satisfaction (including backpack tours by Asia / USA / Africa with many, many domestic flights), and when I see how much dirt, scratches and Co. the Cargo Bag has now, then I am glad that this not on my backpack. The Cargo Bag is very durable, relatively light (220g in the version that I own) and be particularly useful, I feel the extra-mounted clip / plastic holder Aviation Society luggage tags, so that they can not come off.
