I'm partial to the Cayman Sports tips, which would fit your Boxster. Here is a link for them new:
http://e-partssales.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=P&Product_Code=98711198100&Category_Code=ext
Since they are spendy keep an eye on classifieds and E-bay for used ones.
You could also go aftermarket, like Fabspeed, but also pricey
http://www.fabspeed.com/cayman.html