Rack Rollers
When I outfitted my new (to me) truck and box cap a couple of years ago, I got a set of Yakima racks and used some soft plastic high pressure water line (it is clear stuff with fibres criss-crossing through it) which was about 1/4 inch larger inside diameter than the "pipes" on the rack. Slid it on between the towers and it serves the role of allowing me to roll canoes on from the back very easily. Also put a length of it on the front rack for the last positioning. It is flexible enough that it protects the gunnels from any rubbing or abrasion. I also got the wide (6 foot or so) pipes for the racks so I can carry two or three canoes. Used the short, leftover pieces to go on with these pipe on the outside of the towers. The system has worked really well for me -- I leave it on the truck for 9-10 months per year. Have hauled canoes from the maritimes to northern Ontario with it, everything stayed snug and tight, and no abrasion. The flexible water pipe is a bit pricey, about a buck a foot as I recall, but much less than any kind of purchased roller system. I can dig out the details if anyone is interested.
Cheers,
Morley Pinsent