If you are standing the cases up on shelves/ledges, you can keep them from rolling about by using fine (±28ga) brass wire passed through the back through a pair of holes just a touch bigger than the wire, then twisted together.
You'll want the frame to be a bit deeper than the back, just so it will hang neater anyway. Which means the ends of the twisted wire can be kept long enough to undo as needed.
Alternately, you could epoxy brass wire on one side of the case. The ends of the wire could be bent 90º and passed through holes in the back. The wires could then be secured using really tiny brass screws (circa #32 x 3/32") which would simply just hold the wire pinched to the holse in the back. All of which would be small enough to be nearly invisible. Best thing about this approach is that the cartridges could then "float" on the back material, and not need a ledge or shelf at all. This makes labeling a tad easier.
My 2¢