I have a Boot Bag that is basically a soft bag made of tough waterproof material. Used it for a 10 day trip last autumn. That plus bags behind the seats can add up to decent capacity. It sits lower than a solid case so potential to strap an extra suitable soft bag on as well. Being material it is very light so reduces the gross weight. It has built in straps and I use extra ratchet style straps for added fixing.
From configuring a CX, the Morgan rack with side screen storage looks a big unit! I wonder if a Librands one is marginally lighter?
Also, how about some of the motorbike paniers?

Plus Four MY23 Furka Rouge