finally I found a roofrack for the 924S roof rail. Now I can combinate my two passions. The rack is made by Eckel and sold in Porsche Dealership. The back part is mounted with the 4 screws under the glashood. The front part has big rubber feet and clamps to hold it in position. I found my at an open house of the Porsche Zentrum Braunschweig/Germany. The bike carrier is a Mont Blanc barracuda. Below are two closer shots of the mounting.