The supportive sheet metal 211 corresponds in shape to the opening 110 and thus is an oblong sheet metal plate made of stainless steel, but the present disclosure is not limited thereto. For example, the supportive sheet metal 211 may also be provided in the form of any other metal plate of high mechanical strength. The supportive sheet metal 211 has an axle 211A. The axle 211A is pivotally connected to one side of the opening 110. The supportive sheet metal 211 is penetrated by a plurality of through holes 211B. The through holes 211B are equidistantly disposed along the longitudinal direction of the supportive sheet metal 211. In this embodiment, the axle 211A of the supportive sheet metal 211 is further connected to two securing sheet metals 212. The securing sheet metals 212 are fixed to one side of the opening 110 through two screwing elements A to allow the supportive sheet metal 211 to turn over relative to the securing sheet metals 212 through the axle 211A.