I have traveled all over the world with my tripod in my duffle. I use either the Arca Cube head or the RRS 55 head. The tripod fits perfectly in my duffles. I take an old hotel towel that found its way into my bag once. (not sure how) and wrap the tripod with that. If I leave the head on I wrap the head prior to wrapping in the towel with a standard large lens wrap. Both the towel and lens wrap will come in handy when you get to your destination.
As far as duffles go I used to blow through them like there was no tomorrow. So after a lot of research I invested in some not so cheap bags but ones that have lasted and lasted. I ended up with the whole system from Club Glove. They have excellent packing systems, great interlocking pieces for getting though airports and they are tougher than nails. Well worth the investment.
http://www.clubglove.com/CG/ViewProdcss3XTRS-2.asp?ProductID=16 Also, good idea to make sure you have the alan wrenches for your tripod and other tools so you can tighten legs, or work on your tripod or other gear. I place all my tools in a small case in a checked oin bag as TSA sometimes frowns on the transportation of deadly Alan Wrenches in you carry on bags.
Kevin Raber