Try the site below.
http://mike.focke.googlepages.com/towingaboxster
I keep a hard copy of this PDF behind my seats just in case I get stranded somewhere and need to explain to the tow truck driver....
From page 2 of the PDF:
"CAR CARRIER EQUIPMENT IS THE ONLY APPROVED TRANSPORTING METHOD FOR
ALL PORSCHE MODELS."
Hope this points you in the right direction.