A locking retainer for retaining an electronic module or printed circuit card in slots of a chassis. The retainer includes at least three wedge members movably linked to each other and disposed in longitudinal sequence along a longitudinal axis. The retainer can be made relatively larger in height by moving the wedge members longitudinally such that a longitudinal distance between a first wedge member and a last wedge member of the sequence becomes smaller without the wedge members being longitudinally displaced along a common internal longitudinal element. All portions of the locking retainer that the middle wedge member is longitudinally moveable relative to do not extend through a theoretical plane disposed at a longitudinal midpoint of the middle wedge member and oriented normal to the longitudinal axis. The elimination of a central rail permits the wedge segments to have much greater contact area for greater thermal energy flow.