A side rail can define an exterior and an interior channel. A coupling strip within the exterior channel of the side rail can be positioned to selectively engage with a coupling strip of a tonneau cover. An end cap can be positioned on an end of the side rail. The end cap having an end face can include an interior extension within the interior channel and an exterior tab extension within the exterior channel between the longitudinal raised side portions. A retaining fastener can extend through the exterior tab extension and can fix the end cap to the side rail without the retaining fastener passing through the coupling strip. The exterior tab extension can limit longitudinal sliding movement of the coupling strip within the exterior channel.