After years of having my tanks marked by magnetic tank bags, I finally found this little gem. Expensive but works really well and keeps the bag clear of the tank. You get a horse shoe shaped bracket that fits onto the petrol cap and another bracket that you fix to the bottom of the bag (depending where you want the bag to sit). I have only used this on 2 trips so far but it remained perfectly stable, with no buffeting or movement, at 1++ mph. They do lots of options and even have units with power sockets, GPS holders etc.
http://www.motohaus.com/acatalog/Quick-_Lock_Bags_.html worth a few minutes looking... get your credit card ready cos they are bound to have something you want on that site!

