How about a cup holder that goes behind the center console, over the hump in the middle?
You know that middle hump where the rear passengers keep their feet, why not just have 2 floating cup holders in that and have it mounted to the center console?
I doubt many of us will be drinking while driving unless we're on the highway.
Something like this
Except mounted behind the center console
Maybe instead of having to drill new holes, we just use the hinge for the center console cover and we might need to shave some off for clearance for your brackets