an uppermost u-shaped lateral tube having a solid top and two sides forming a lengthwise opening therebetween;

a plurality of lower u-shaped lateral tubes each having a top with a plurality of openings and two sides forming a lengthwise opening therebetween;

two lower support ridges extending inwardly and horizontally from two opposite sides of said lengthwise opening, wherein each said lower support ridge has a convex surface;

two upper stoppers extending inwardly and horizontally from two opposite inner walls of said two sides just above each said lower support ridge;

a plurality of upright tubes passing through said lengthwise opening and said plurality of openings in said lateral tubes;

an elongate, plate-shaped position strip connecting each said lateral tube, said position strip comprising:

an upright wall, wherein a back side of said upright wall is located flush against said plurality of upright tubes and a front side of said upright wall is located in an area between said lower support ridge and said upper stopper of each said lateral tube, a height of said upright wall being equal to a distance between each said lower support ridge and each said upper stopper and a thickness of said upright wall being equal to a distance at which said lower support ridge and said upper stopper extend inwardly and horizontally from said openings and said inner walls, respectively such that at least one end tip of said upper stopper or said lower support ridge contacts an outer surface of each of said upright tubes; and

wherein an upper surface of said front side of said upright wall has a convex surface and a lower surface of said front side of said upright wall has a recessed surface; and

wherein an intermediate portion of said front side of said upright wall has a lengthwise recess; and

a plurality of screws, wherein said screws pass through said elongate, plate-shaped position strip and said upright tubes to secure said position strip to said upright tubes.