a plurality of ribs 124 protrude from the bottom panel 12 the ribs 124 surround and support the fixing pole 122 on the bottom panel 12 the fixing pole 122 defines a fixing hole 1222 for mounting the nut 20 to facilitate the mounting of the nut 20 in the fixing hole 1222 the fixing hole 1222 may be slightly larger than the nut 20 the nut 20 is a hollow cylinder and includes a first end 21 a second end 22 and a peripheral surface 24 connecting the first end 21 and the second end 22 the nut 20 defines a longitudinal threaded through hole 222 a first flange 244 protrudes from the peripheral surface 24 at the first end 21 and a second flange 246 is at the second end 22 the peripheral surface 24 further defines a receiving space 242 which includes a plurality of flutes or grooves 2422 longitudinally defined the receiving space 242 retains the glue 30 the first flange 244 and the second flange 246 both retain the glue from escaping and spilling during curing to retain the nut 20 precisely and permanently in the fixing hole 1222 referring to fig2 4 in assembly the glue is applied to the grooves 2422 the glue 30 may be quick drying the nut 20 is quickly inserted in the fixing hole 1222 the glue 30 is cured.