BilgeRat
Elio Addict
Maybe one of the after market supplier will design a luggage rack that can be installed on the trunk.
You might not even need the luggage rack. I have a pair of rectangular waterproof bags by Atlantic Design that I bought for my Miata. The two bags clip together with Fastex buckles, a pad goes over the trunk deck and the whole works is fastend down with adjustable straps. It's not necessarily Miata-specific, either. The assembly worked equally well on my wife's Geo Prizm when she had it; we used the bags for trips to her folks in North Carolina, and they worked very well.