You really only have 3 options. Two of them, mentioned above are the Williwood trailer hitch and the Bird rack. The Williwood one requires some modification the rear bumper, the Bird rack, requires permanently mounting tubes under the car to attach/detach the rack.
The third option is the Porsche Roof Transport System (RTS). I have one, which I use with a ski box, and my surfboard. It is the only solution that does not require any modifications. If you can find one used they normally run about $300-400, and you can use a regular roof mounted bike rack.
RTS Accessories
Boxster S
Genesis 3.8
2002 Porsche Boxter S
2001 Porsche Boxster S
1999 Porsche Boxster
1999 Porsche Boxster
1999 Porsche Boxster
1999 Porsche Boxster
2000 Porsche Boxster
1999 Porsche 996 C4
1972 Porsche 914
1984 Porsche 928S
1984 Porsche 928S
1985 BMW 635 Csi
Porscha
Linear Mode
