In reply to pasbury:
Like nniff I also use Sea Sucker racks (for Mini Cooper). Very unnerving at first use, but have found them pretty bombproof, security when unattended being the only issue.
Using the rack designed for paddleboards/surfboards to carry a Dagger Stratos 14.5L kayak which I think weighs in at about 24kg dry. Only 30/40 mile trips so far, including motorway driving, but the kayak has never shifted and even over a couple of hours there has been no loss of vacuum in any of the six suckers.
High initial cost so keep an eye on the second hand market.