Most hard bags are designed to latch to a special-purpose frame that stabilizes the box in all axes. This means at least three attachment points.
It's certainly possible to bend aluminum bar stock to form hooks that can bolt to the box and hang from the grab rail (I've done this), but you're left with the problem of preventing side swing -- you'll have to secure the bottom of the box to the passenger footpeg or muffler hanger, or something. Workable cheap solution if you use tough plastic tool boxes for panniers, or ammo cans, or Pelican cases.
