Abstract: An interlocking male member is provided in a lower end portion of each side member. The interlocking male member has a projection extending beyond an outer surface of said side member and a bulge on its vertical wall which projects inwardly away from the projection. An interlocking female member is provided on an periphery of a bottom member to extend along at least portions thereof which correspond in position to the male members of the side members. The female member has a locking hole in which the projection of the male member is locked, and a recessed portion in which the bulges are locked. The side members are mounted to the bottom member in an upright position, by fitting the male members in the female member, while tiling the side members with respect to the bottom member, so that the projections are locked in the locking holes, and then rotating the side members toward the upright position so that the bulges are locked in the recessed portions.