A casing member for a WDM add/drop multiplexer unit, the casing member comprising a backplane for interconnection of components of the WDM add/drop multiplexer unit inserted in the casing member, and at least one heat sink opening formed in a wall of the casing member disposed to, in use, receive a heat sink structure of a component of the WDM add/drop multiplexer unit in a manner such that the heat sink structure is exposed to an ambient around the casing member when the component is mounted in the casing member, for facilitating maintaining a controlled temperature environment inside of the component.