I have a Cortech Super Mini tank bag and just love it. It is well made and has the best base material I have seen on a tank bag. It will not scratch the tank. The magnets are strong and there is a row of them on the base of the bag as well, so it does not jiggle around. It is 6.5 liters and fits the bike perfectly. You can still tuck down behind the windscreen which is really important IMHO. I use that along with a Fieldsheer expander tailbag, which also fits perfectly, and does not scratch anything.
Amazon.com: Cortech Luggage - Cortech Super Mini Tank Bag: Automotive