Abstract: A casing includes a hollow rectangular main body with upper and lower walls and front and rear upright walls to define right and left openings, and two pairs of mounting seats fixed to and disposed inboard to the upright walls and extending a longitudinal direction of the upper and lower walls. A retaining member is disposed above the upper wall to define a retaining groove. Each of two side plates has a depressing force transmitting abutment portion extending from an upper edge in a transverse direction thereof, and front and rear lateral abutment portions for abutment with the mounting seats when brought to cover the openings. A cover plate is superimposed on the upper wall, and has a front cover edge and a rear cover edge insertable into the retaining groove.