We need a Messenger Bag Packing Cube.
It would be similar to the shoulder bag packing cube--with two clips to attach a shoulder strap--but much larger in size. Closer in size to the backpack packing cubes. It should be large enough to hold a Macbook Air. (In a cache, of course.)
At least one "o" ring, if not more, in each compartment. Dual zippers for on all openings.
While I'm no designer, I don't see why TB can't just take a backpack packing cube, remove the backpack straps and add two clips just like the shoulder bag packing cube. (which, by the way, is just too small for my liking or needs.)
I prefer hands free movement and like messenger bags over backpacks.