the easiest route might be to get a Yakima roof mount kit, there are some ideas/pics in this thread that might get the gears turnin for you
http://www.escape-city.com/viewtopic.php?f=11&t=4772&p=85579&hilit=yakima#p85178
http://www.escape-city.com/viewtopic.php?f=11&t=4772&p=85579&hilit=yakima#p85178