That requires putting the boat on stern towards the car. I personally now do that with my cat and like it... but it sure took a leap of faith to lower my mast over my car the first few times...

If you dont want to do that... i used to use a milk crate with a life vest(or even simply a cushon) on it to lower the mast onto (on both my hobie 16 and 18). It lasts for a few years. I would use several heavy bungies and secure the crate to the tramp and then the mast to the crate for transit.