They're ugly, but every now and then I need to put my surfboard on my 928 to go surfing, so I put some surf racks on. First I bought OEM roof racks from a local dealer (like the ones at 928 Specialists).

The racks install in minutes as the fronts simply screw onto the rain gutter and the rears bolt into factory threads at the hatch.

Then I bought a 'Thule 554 Hang Two Surf Carrier', which is really 4 plastic board holders that attach to Thule's racks. (they also come with 4 rubber straps you use to strap down the boards)

These will fit the Porsche OEM racks by using inserting couple washers inside the rack, but I filed down the bottoms of them to prevent whistling. Here's a closeup of where I filed them down.

The knobs allow you to move side to side on the rack for a better fit and re-tighten.

Here's a couple shots of them with the surfboard fitted. They're awesome because I can actually stack two boards on top of each in seconds.