I have something much like ST is suggesting except the whole unit is integral and ready to use with telescoping side bars. Works supremely well IMO.

I think I got mine from Piragis some 10 years ago and picked up the suction cup front rack as well as another rack which mounts into my trucks receiver hitch.......slick set up. I know the entire shebang was around $200 but now they want $180 per unit (ouch). Lately I have just been using the front suction cup rack and using some foam blocks for the rear. Ropes via truckers hitch secure the boat to the vehicle in addition to being secured to the rack. Good set up which is easily removable.......another plus :thumbup