An adjustable clamp assembly for attaching an accessory to a vehicle having a cargo rail. The assembly includes a rail clamp member attachable to the cargo rail and an adjustable clasp catcher operatively attached to the rail clamp member. A clasp is configured to selectively interface with one of at least two attachment features on the adjustable clasp catcher. An accessory fitting configured to accept attachment of the vehicle accessory is connected with respect to the clasp such that the accessory fitting is selectively attachable to the adjustable clasp catcher, thereby capable of attaching the accessory to the vehicle through the cargo rail. A locking member may be operatively attached to the clasp and configured to selectively exert a locking force between the clasp and adjustable clasp catcher.