an upper case including a coupling plate protruding along an outer circumferential surface of the upper case and a plurality of coupling holes formed in the coupling plate to be spaced apart from each other;

a lower case in which a connecting plate having a shape corresponding to the coupling plate and including a weld groove formed by partial engraving is formed to protrude from an outer circumferential surface of the lower case;

a weld nut which is disposed between and contacting the upper case and the lower case to maintain an axial force and is welded to the weld groove of the connecting plate;

an assembly bolt which passes through the coupling hole of the upper case and is insertion-coupled to the weld nut; and

a gasket which seals a space between the upper case and the lower case, wherein the gasket is disposed between and coupled to the upper case and the lower case in a snap-fit manner.