and the right board 5 are the same the assembling configuration of the left board will be described below as an example as shown in fig4 to 18 the assembling configuration of the left board 4 and the baseboard 1 is as follows a horizontal retaining groove 7 of a front rear direction with an inward opening is formed at the bottom of the left board 4 in the present embodiment the horizontal retaining groove 7 is a t shape groove with the rear end closed a horizontal rib 8 of a front rear direction capable of forming an insert connection with the horizontal retaining groove 7 is formed on the left side of the baseboard 1 consequentially a first insert connection structure is formed between the left board 4 and the baseboard 1 limiting the left board 4 to only slide frontward and backward relative to the baseboard 1 during installation the left board 4 is inserted onto the baseboard 1 in a front rear direction the left board 4 is installed in place when the closed ends of the horizontal rib 8 and the horizontal retaining groove 7 abut on each other the assembling configuration of the left board 4 and the front board 2 is as follows a vertical retaining groove 9 of an up down direction with an inward opening is formed at each of the front and rear sides of the left board 4 in the present embodiment the.