Thought I would throw my 2 cents in.....I have a Cortech Dryver Tank Bag. They come in a couple different sizes and snap into a Ring Lock mounted on your gas filler ring.
I only needed the small one to carry ferry money, sunglasses, a snack for the road and my heated jacket controller when I'm not using it. It is completely waterproof and for a little extra you can order it with a map case. Their website shows that you can buy the ring that mounts on the tank for several brands of bikes so you can move the bag from bike to bike. Easy to remove and put back on.