Paul, i made a very cheap and very inexpensive roof rack for my car using the foam block idea and scrap 2x4's. The 2x4 was cut to two appropriate lengths(depends on your car) and the foam blocks were attached near the ends using rubber cement. A cradle was laminated up using the deck forms from my boat and attched to the 2x4. The "racks" were then simply placed on the roof and the boat fitted into the cradles. using the rachet straps u already have fasten down the boat/rack combo to the roof. This method has lasted for hundreds of miles and though several hail producing t-storms without incident.

P.S. Lock them in your car, somebody liked my first set so much they stole them off the roof of my car. It was not a pleasent return trip home needless to say.