Upper assembly including slot, hole array. Lower assembly conforming to hips. Lower assembly anchoring holes shaped/spaced as array. Lower assembly fastener hole for each slot fastener hole having same position relative to anchoring holes as slot to array having horizontal dimension equal to horizontal dimension of corresponding slot. Adjuster plate including anchoring points spaced as array holes. Each anchoring point dimensioned less than corresponding hole so as to fit snugly therein, and projecting a distance equal to thickness of corresponding upper and lower assembly anchoring hole. Forming an adjuster plate fastener hole for each lower assembly fastener hole. Each adjuster plate fastener hole having same position relative to anchoring points as slot to array. Releasable fastener for each lower assembly fastener hole. As assembled, fastener secures adjuster plate to upper and lower assembly such that anchoring points of adjuster plate are positioned through upper assembly and lower assembly anchoring holes.
