I had used foam blocks and Yakima racks for years, then I got a car which could not mount a rack. My sons and wife came came to the rescue that Xmas with help from Rutabaga. They found the Malone handy rack inflatable car top carrier.
It looks like two WWII G.I. life belts, but works amazingly well. It is strapped on the car top by threading the tie-downs through the open doors and then inflated. I have used it for my Old Town Discovery 164 and Wenonah Fusion. It comes with all straps and air pump. I like it almost as much as my old Yakima. There is only one draw back. When it rains the tie downs act as wicks resulting in showers inside the car. If you can't use a regular rack, or your car doesn't have a factory rack, this is the way to go.