A deflector member may include a front portion positioned on the front side of the opening and corner portions extending, in a curved manner, rearward from both ends of the front portion, respectively. A lower edge of the deflector member may form a notch between the front portion and the corner portion. The lower edge member can be divided into a front lower edge member attached to a lower edge of the front portion, and an arc-shaped lower edge member attached to a lower edge of the corner portion. The frame structure may be provided with a front attachment body and an arc-shaped attachment body to which the front lower edge member and the arc-shaped lower edge member are detachably attached, respectively.