A coupler comprises a first channel having a first axis of extension in a first horizontal direction and a second channel having a second axis of extension in the first horizontal direction. The second axis of extension is parallel to and spaced apart from the first axis of extension in a second horizontal direction. The second axis of extension has a pitch angle relative to the first axis of extension in a vertical direction, the pitch angle being less than 180 degrees. The first and second channels each include head-mounting features, the head-mounting features constructed and arranged to retain an elongated head to the coupler while permitting movement of the elongated head relative to the coupler in the first horizontal direction.