I've decided to trade my Z4 35is on the new Porsche Boxster S and about 2 months ago bought the Roadster Solutions rear wind block. There are no marks/scratches. So if you were considering one (they work great, I was very pleased with it) and want to save $100+ here's your opportunity.
Asking $150. I'll cover shipping in the U.S.
You will need to get some 3M foam tape, which can be found at most good auto parts stores and of course online.
http://www.roadstersolutions.com/page69.html
Scott