Abstract: An outer case including a rectangular base having four upright side walls of different heights; pairs of supports respectively hinged to two opposite ends of each of the four upright side walls of the base by hinges; a plurality of mounting blocks respectively fastened to the supports and arranged in stack on each support, each mounting block having a center through hole for the insertion of one support, a mounting groove at one end, and a sliding groove at an opposite end; a plurality of wedges respectively fitted into the mounting grooves of the mounting blocks to fastened them together and to secure the supports in vertical; and a plurality of transverse panels respectively fastened to the mounting blocks between each pair of supports to close the four sides of the outer case, each transverse panel having two opposite ends respectively fitted into the sliding grooves of the mounting blocks; a top cover covered on the outer case at the top side; and a circular outlet made on one upright side wall of the bas