try ringing up Gemballa in Malibu 310-457-1709
or post this thread on Rennlist and flatsix.
Also try RUF North America in Texas, those guys will talk your ear off.
Good luck I've been cosidering the aftermarket diffuser, if filling in the gap along the seems can be done well.
http://www.germancarfans.com/manufacturers/porsche/index_preview.cfm