If your going to install a ski rack on your sled then look at these. No sled company builds their tunnels with the idea of putting a lot of weight on the tunnel. Skis, boards and a pack put a lot of force on the tunnel. Here is a video on their install.
http://www.youtube.com/watch?v=mfge30rX2j8
http://www.youtube.com/watch?v=mfge30rX2j8