We prefer these simple nylon covered foam pads to expensive roof rack cradles or rollers. They slip over Yakima, Thule, and most narrow factory racks. Velcro tabs and tie downs are sewn in, so they won’t blow off.
At 18" in length 2 pair will fit side by side on a 50" cross bar. For wider factory type roof racks order the "Aero Rack Pads" as shown below.
Available in a 30" inch pad for your wider boats.