Howdy,
Owing to the variety of B's with A/C/ units, vents, antennas placed every-which-way, I think you're going to have to make your own.
A suggestion, Look in Walmart for "Pool Noodles", These are foam tubes that look like pipe insulation, only more colorful
Good padding for the canoe but keep in mind that the foam used is not real UV resistant. I'd wrap them in duct tape or similar if the RV is left in the sun with the rack on.
Chip