A long time ago I got some foam blocks from L. L. Bean which were designed to fit on to the gunnels of a canoe so you could carry it without roof racks. I took a piece of pipe and those blocks and made my own roof rack to carry kayaks. I also use it to carry wood, furniture, and anything else I cannot fit into the car. Later on today I plan to mount them and use them again.

Get some good solid foam rubber, not the stuff they make mattresses out of but something that has some solidity. You want a block about six inches to the side. Get a length of pipe that will reach across the width of your car. Carve the foam to accept the pipe. The best way would be to cut a hole through the foam and insert the pipe so the two items become one unit. Use a flat strap around the ends of the pipe and meeting in the car. Tighten it and go.

I made mine wider than the car because I wanted to get three kayaks up there. After a couple of painful bumps I added foam to the pipes. I used a Fun Noodle (lengths of foam they sell for kids to play with). It's brightly colored and there is a hole in the middle you can force the pipe through. A short piece on the ends of each pipe does the trick.

Note, the Fun Noodle should not be used for the whole length of the pipe because it adds too much diameter and presses on the center of the roof. The foam used to make those toys is about the right consistency for the blocks of foam needed to support the pipe but they are too small. Maybe with some glue or something you could build up a decent block.

Do not try to use electrical conduit or PVC for your pipe. It is not tough enough and will bend under the load. Get some sturdy 1/2 inch or 3/4 inch water line pipe.