If you have a Rhino-Rack Heavy Duty Crossbar system or a Prorack Trade Crossbar and would like to mount Rhino-Rack's range of Kayak and Canoe Carriers including the Nautic 580 and Nautic 581, you will require the HD-FK2 Fitting kit. The kit comes with all the necessary bolts and washers to attach the kayak carriers to your Rhino-Rack Heavy Duty Bars or Prorack Trade Crossbars.