Bag for long mortorcycle trip

I'm a Leica M photographer looking for a bag that will MAXIMIZE the interior compartment of one of the side cases on my bike, (approx, but not larger than) 18Lx8Wx12H. I have 2 bodies, many lenses, flash, etc. Would also be great if it had back straps for some hiking, but not essential. Thanks!

[2mnycars]

Hate to say it here...vibration would be a concern for me as an x-biker.

Perhaps Mr. Domke can contribute; he has so much experience that I don't.
I'm an X M3 and M4P owner. I would have considered a chest pack or a fanny pack.

Good luck! Keep the shiny side up, and the greasy side down!

DaveL