the second folding sidewall 13 are formed by two panels which are pivotly connected together therefore by this structure the first folding sidewall 12 and the second folding sidewall 13 are foldable toward to the back wall 11 to drive the front frame 14 to flatly contact to the back wall 11 please see fig3 the space occupied is decreased to save transport costs or save the storage spaces when unfolding please see fig4 the back wall 11 and the front frame 14 are moved far from each other until the first folding sidewall 12 and the second folding sidewall 13 are unfolded see fig2 also the first folding sidewall 12 and the second folding sidewall 13 have studs 50 at a top side 122 132 and a bottom side 123 133 of an inner surface 121 131 the top panel 20 includes notches 40 at two opposite sides 22 23 of a bottom surface 21 and the bottom panel 30 includes notches 40 at two opposite sides 32 33 of a bottom surface 31 please see fig5 the top panel 20 and the bottom panel 30 are connected to the folding frame 10 between the first folding sidewall 12 and the second folding sidewall 13 near the top sides 122 132 of the sidewalls 12 13.