You could buy this Duraflex rear bumper, cut it out and use mesh like in the picture.
[IMG]http://www.****************************************/images/duraflex/body-kits/104994-2.jpg[/IMG]
1997 2004 Porsche Boxster Duraflex Maston Rear Bumper Body Kit | eBay
I bought one to put on my car, unfortunately it arrived damaged. I can tell you this though while it was in my hands. They don't do a great job on the fiberglass laying. Will need lots of prep work.