and fixed to the wall or window frame through two self tapping screws 1 and the inner side thereof adheres to the wall or window frame the outer side 2 1 of the stationary seat 2 comprises two rows of longitudinal teeth 2 2 in which each tooth has an upper part shaped as an upward slope and a lower part shaped as a downward plane two longitudinal edges of the outer side 2 1 continuously turn by 90 degrees twice respectively to form two longitudinal chutes 2 3 which are faced opposite to each other two longitudinal chutes 3 6 are formed on one side of the bottom cover 3 and two projecting flexible blocks 3 5 two longitudinal chutes 3 6 of the bottom cover 3 are matched with two chutes 2 3 of the stationary seat 2 in particular the projecting edges adjacent to the chutes 3 6 of the bottom cover are positioned into the chutes 2 3 of the stationary seat 2 and the projecting edges adjacent to the chutes 2 3 of the stationary seat are located into the chutes 3 6 of the bottom cover 3 so that the bottom cover 3 is slidably adapted to the stationary seat 2 the flexible blocks 3 5.