The present invention relates to an apparatus, releasably attachable to a rail, having a holder and a receptacle for receiving and releasably attaching to the rail to support the holder on the rail, wherein the holder includes an outboard portion and an inboard portion, wherein the outboard portion lies beyond the rail and the inboard portion lies within the rail, when the holder is supported on the rail, and wherein the outboard portion is lighter than the inboard portion, so that the apparatus is weight-biased within the rail, such that the apparatus is adapted to resist dislodgement by outboard forces and to tend to dislodge in an inboard direction.