Thanks for the reply.There's no way I'm going to leave any composite boat on top of my vehicle to bake in the sun for any length of time, especially the month of July. I would go to the fabric store and sew a simple tube out of fabric to slide over it if I didn't have or couldn't afford a custom cover. Looking at the cost of a new carbon canoe, I don't see why you'd take that chance.
I completely agree with you that it is stupid to risk anything with such an expensive canoe.
I have now gotten a cover for it and hope it is UV resistant.