This is off the wall, but we are also trying to figure out how to carry a kayak with us and have seen arrangements like this on a number of motorhomes and have been trying to figure out if there is a way to fab one that would connect to the hitch on our truck and the camper roof.
I’ve seen this used successfully on RVs. Not an option for me since my boat is 17′ long.
The one concern I’d have about putting one on a relatively short TC is that if you have much boat sticking out, the aerodynamic forces could be massive. Also, this setup would be heavy if weight is a concern.