More cargo capacity for a c6 convertible

I want a 2008/2009 convertible, but I need a tad more carrying
capacity than the miata-sized trunk. How can I find a way to carry a
couple more suitcases?

I know the coupe has a nice large volume of cargo space underneath the
hatch, but I have had a coupe before, and now I want a convertible.
My problem is that I need to travel once a month or so with the old
lady out to see the grandkids,.. a 10 hour ride. To take the stuff we
(she) requires for the trip, the convertible just doesn't have enough
space in the trunk. The coupe has plenty.

Got any ideas? What possible way is there to carry a little more
stuff? I had luggage racks on a Miata, and an MGB years ago. A Miata
buddy had a trailer hitch on his car to tow a little trailer to haul
race wheels to events, and he suggests using some sort of platform
plugged into a receiver hitch.

Any info will be greatly appreciated.

mel