Rack systems are disclosed for coupling accessories to crossbars on top of a vehicle. A coupling device may include a flexible strap member adjustably anchored at one end of a body. The strap member may be connected to the other end of the body and tightened. A mounting assembly may be provided on the top side of the body for connection to a cargo-specific carrier or accessory. A coupling device may also include a flexible fitting member configured to grip a round or rectangular crossbar. A cap member may be configured to be mounted on the fitting member and have one or more ramp surfaces which form a sliding interface with a corresponding surface on the fitting member, thereby providing a closing or tightening effect on the fitting member with respect to the crossbar. Other coupling devices are configured to grip or clamp a t-slot in the crossbar.