The base unit further preferably comprises depressions or slots, into which fasteners such as rubber bands or strips tightenable with Velcro.TM. may be seatedover the ends of the extensible sheath to further prevent unwanted slippage of the extensible sheath.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1A is side view of the preferred embodiment of the invention, without the extensible sheath in place.FI