If a new skin is not available, there are two other options:
Get a deck lid from a junkyard if rust hasn't eaten holes thru the outer skin. Use a putty knife to scrape off the tar/asphalt coating on underside of deck lid, then send it to a dunk tank. When you get it back from them, the deck lid will be coated with metal prep which must be washed off with a solvent prior to primer coat. After flushing the metal prep, turn the deck lid upside down and dump a thinned coat of marine primer or acid etch primer onto the underside of deck lid so that it runs down under the metal structure. Turn the deck lid so that the bottom of the lid is horizontal and slosh the primer around to make sure the primer seeps in to lower corners. Once you are satisfied that the underside of the skin is thoroughly coated, turn the deck lid right-side up and drain the excess primer into a large open can or bucket thru one of the holes near where the hinges bolt to. This may help to protect underside of skin from rusting due to condensation ever again.
In lieu of getting another deck lid, see if you can salvage your old one by doing same as above, but if there are already holes ate thru at the lower corners of the deck lid, you may need to cut those off and rebuild those corners with new steel. The curves will be difficult to duplicate though.