Watch the weight. It's a challenge to build a boat light enough to lift on
and off the roof of a car yet large enough to be of much use. Canoes
and kayaks are popular because they are light. Decide before hand if you
will be doing the lifting yourself or have a helper. That will govern the
size and weight of boat you can practically carry on the roof of a car.
Some people make gin pole hoists so they can lift bigger boats solo.