You got me there Dave. I've always used the classic North Face bags for transport on aircraft, trains and mules etc. For our last Patagonia trip the large (or was it XL?

) version was able to take my mountain rucksack and all personal expedition and climbing gear for a 3 week trip (excluding food which we bought locally).
I have never had one of these bags break. I do put a separate padlock on the though. Agreed, they are a touch on the heavy side. Let me know if you come across better options.