Yakima BigStack Kayak Stacker in great condition. I've used it to carry 3 kayaks and a paddleboard on the roof of my SUV. Comes with heavy duty straps, bow and stern tie downs (purchased new last year for $50 and never opened), and two Inno INA451 cradle-stoppers (a $60 value).
I never installed locking cylinders, but they can be purchased from Yakima.