a flexible cover for mounting over the cargo area of a vehicle such as a pick - up truck , boat or the like , having resilient , j - shaped extruded fasteners attached along the edges of the cover for fastening it to outwardly extending longitudinal edges on rails on the vehicle sidewalls . the j - shaped fasteners are formed of a resilient plastic which is mated with the rails in such a manner as to accommodate variations in the distance between the cargo box sidewalls , as well as variations in the flexibility of the flexible cover . the extruded members are adapted to be snapped together by applying a downward force on the j - shaped fasteners toward the rail until the edge of the rail snaps into the resilient fastener . the cover is removed by unzipping the j - shaped fastener from the rail to expose the cargo box area . to allow the covers to be put in place amd kept taut in both hot and cold weather , one of the rails can be provided with multiple parallel edges defining a channel between each pair of the edges so that one fastener can be brought into engagement with whichever edge will keep the cover taut in the direction perpendicular to that rail . the fastener along one edge can be t - shaped and slid longitudinally into engagement with the appropriate edge .