by nine slots 29 cut at an angle of thirty degrees each of the slots is about one and a half inches deep and one sixteenth inch wide measured along the upper surface of the inverted u channel 9 the slots 29 are spaced about three and one quarter inches apart the nine slats 4 each include a body part 31 a head part 33 extending the full width of the body part 31 and a tail part 35 extending less than the full width of the body part the length of the tail part is chosen to be equal to distance between the inside faces of the vertical frame members preferably the line between the body part and the tail part is partially undercut to permit the tail part to be cut easily to the appropriate length the head part is bent forty five degrees from the plane of the body part toward the frame 3 and the tail part is bent forty five degrees from the plane of the body part away from the frame 3 each slat is made of one sixteenth inch coated steel twenty nine gauge and has a length of forty inches the body part is three inches high the head part is one inch high and the tail part is five eighths inch high the overall height of the slat is about four and one eighth inches it will be seen that the body parts of the slats 4 are about ten degrees from true vertical it will be seen that.