There was a post about 2 months ago about using pipe insulation cut in half, then wrapped around the yak.

I did this method, and it worked great. I bought 6 feet of pipe insulation, and cut it down the middle. I wrap it around the kayak with a bungee cord (Convex side toward the yak) and throw it up on the car. I then strap it down, and tie it front and back. Works really well, and it feels very secure.