the base 10 the two connecting bars 40 are installed one hinge end 42 of each connecting bar 40 is placed into the second connecting groove 124 of the base 10 to rotatably connect the connecting bars 40 with the base 10 then the cover 20 is assembled with each second hinge portions 342 being inserted respectively into one of the first receiving grooves 222 to rotatably connect the swinging plate 30 with the cover 20 the other hinge end 42 of the connecting bar 40 is positioned into the second receiving groove 224 of the cover 20 to rotatably connect the connecting bar 40 with the cover 20 the cover 20 is closed and each extending arm 34 is received in an area defined by the first connecting groove 122 of the base 10 and the first receiving groove 222 of the cover 20 each connecting bar 40 is received in an area defined by the second connecting groove 124 and the second receiving groove 224 the elastic member 50 is in an unstressed state where the end portion 54 extends into the resisting area 2262 and resists against one of the stopping blocks 226 referring to fig4 to open the cover 20 the cover 20 can be pushed with an external force to slide along the upper surface 12 of the base 10 allowing the two connecting bars 40 and the swinging plate 30 to rotate as a integral unit and accordingly tilt the cover 20 at the same time the elastic member 50 rotates along with the swinging plate 30 the end portion 54 of the elastic.