Duffel bags are part of any big trip, carried on the roof of trucks and mules and porters, getting all your gear from A to B; but when it comes to flying, it’s worth checking the actual weight of your bag. The early models were based on navy duffels (Seabags), which were big, tough mail sacks, that took up very little space once empty, and were cheap and simple. Now such bags are like lifestyle statements, made out of haul bag vinyl, with heavy-duty zippers, mesh pockets, all of which add up to a lot of weight (a $200 XL North Face Duffel weights over 2kg while a $20 army surplus duffle weighs just 1kg). Another option is to buy a roll of Cordura fabric off eBay (500 to 100 denier), and just make your own bag: like a mailbag, with drawcord closure (use a heavy-duty cord and tie it closed). Keep the bag under 130cm in length (the same as a golf bag); otherwise, you end up with problems at check-in (people will start measuring things and ringing supervisors, who will ring their supervisors etc.). An alternative to this is something like the @aiguillealpine giant stuff sack/travel tote, which is cheap, light and can hold 120 litres of holiday shite.