Just place the backpack on the tunnel and hold it in place with your legs. Sounds dodgy, but it works really well. If I go shopping on the way home I place the shopping under the seat and to make space, my work backpack gets moved from under the seat and sits on the tunnel. If you are goig for the topcase option, a Shad or Givi is usually a much cheaper option than factory accessory, and of the same quality. And there are more style and size options.