Sorry for the long winded post but I think it is worthwhile.
From what I have discovered in researching the fascia kit from Porsche mentioned above you will still need: mounting hardware to support the head unit, you will need wire harness adapters (unless you want to t-tap or cut into the factory harness) and you will need to paint-match the Porsche fascia part. I also cannot confirm it will work with aftermarket head units.
Way back when I started researching information for a head unit replacement project I knew about the "Japan" kit but had time to research more. I looked into factory parts and options but I had a hard time finding the Porsche fascia or for that matter any mounting hardware to properly secure an aftermarket head unit. I even spent half a day at SEMA talking to every aftermarket adapter manufacturer at the show to see if anything was on the horizon for development... Nothing... Just a lot of "If you have a new Porsche you should just have a custom shop install your radio" My thought: No way! I'm an electrical engineer with a major case of CDO (That's OCD in alphabetical order!!) and to boot I enjoy do it yourself projects, especially when it comes to my Cayman.
During my digging around I found the following post on Renntech: New DVD Navigation Install - RennTech.org Forums
When I saw the work they put into developing something for the Boxter This prompted me to contact the guys in Bradenton Florida directly since they are 40 minutes from me... They in turn built a kit for the late model Porsche and used my car as the test vehicle. It was a prefect scenario.
I used the kit from the web site below for an AVIC D3 and BartD above used it for a Kenwood head unit. It does work with aftermarket head units.
My kit came from eBay Store - Euromotorspeed: Interior Parts, Exterior Styling, Car Audio
they are local to me in Florida and everything needed was included to trim the aftermarket head unit, wiring adapter, antenna adapter, mounting brackets, screws, and detailed instructions (they posted instructions as well on another thread on this forum, my car was their test vehicle)
I also added a JL audio amplifier to replace the Bose amp... I'm working on a write up this week on that...
Hope this helps...