There are several streamlined car carriers to carry all sorts of gear. I think it would be feasible to create a plastic or fiberglas carrier that could double as a kayak.
There could be a lockable, outside shell or hatch to secure the carrier. The hull would be one piece with an attachable bow and rudder, kept inside.
After lifting if off the car and removing the contents, total assembly time would be about 10 minutes.
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