Have you consider re-deploying a shelf from industrial wire rack shelving? These are often 18 inches wide (45 cm), and up to 72 inches long (183 cm). Very much like a Yakima Basket Case. A long one might need more supports than a typical two-bar rack, but the shorter ones might work quite well. The clips from Thule or Yakima might be adapted easily to attach, or it wouldn't be hard to craft a u-bolt setup to tie it to the rails.
Often available *cheap* at places that sell used commercial furniture. Usually chrome, which would probably need to be coated for exterior use.