The Yakima Evenkeel clamps right onto the factory crossbars, and are adjustable in width. I use it for my boat, a 14ft pungo, which weighs 50lbs or so. I can just throw it up there myself.
A cradle may be a better idea. I had bad luck with foam blocks. The plastic on the rounded crossbars made for a lot of slipping, especially at speed and turning. I've got a Yakima saddle and its much better.
