A lockable retainer assembly adapted for retaining or releasing a panel having at least one edge by preventing or allowing movement of an edge of the panel. The lockable retainer assembly includes a rocker rotatably disposed at the edge, the rocker having a slot and is configured to protrude into the panel's path of movement when the rocker is rotatably disposed in a relaxed position. The rocker is configured to clear the panel's path of movement when the rocker is rotatably disposed in a depressed position. A lock is slidingly disposed within the slot and adapted to prevent rotation of the rocker when the lock is disposed in a relaxed position and allow rotation of the rocker when the lock is disposed in a depressed position. In the depressed position, the clears the path of movement of the panel, enabling the panel to be inserted or removed through the path.