Windsurfing Boom Want To Transport 12' Windsurfing Board/boom Without Installing Roof Rack; What's Best Method?

Want to transport 12' windsurfing board/boom without installing roof rack; what's best method? - windsurfing boom

I take a surfboard 12 '/ Boom (full rig, but 2 to 12 m long), but I have a Mazda Protege and MINI Cooper, so I mounted on the roof. What are my best options if you do not want to install a permanent roof rack? Can I foam / inflatable rack locally (Wal-Mart, Sports Authority, etc., etc.) I have a board or cardboard and string to use / straps through the windows secure?

Thank you.