the top plate having a central through hole and connecting parts on its lower surface corresponding in position to the respective upright poles;

an upper end face of each upright pole being rotatably connected to a corresponding one of the connecting parts in a snap-on manner, each upright pole having mounting grooves on its sides, the mounting grooves each extending downwardly along an axial direction of the upright pole and reaching a lower end face of the upright pole;

two ends of each side plate being insertedly connected to the mounting grooves of corresponding two of the upright poles from lower ends of the corresponding upright poles;

the limiting members being movably connected to lower end portions of the sides of the respective upright poles for limiting and preventing the side plates from falling out of the mounting grooves of the upright poles;

the connecting parts each include a connecting post welded to the lower surface of the top plate and a connecting sheet welded to a lower end face of the connecting post, and the upper end face of each upright pole is rotatably connected to the connecting sheet in a snap-on manner;

the connecting sheet has a perforation, a limiting groove communicating with the perforation, and an arcuate notch; the limiting groove is located at a center of an imaginary circle defined by a circumferential contour of the arcuate notch, the upper end face of each upright pole is provided with two fasteners, and a distance between the two fasteners is equal to a radius of the imaginary circle defined by the circumferential contour of the arcuate notch.