near bottom end 21 of frame 9 the exposing of inserts 24 a allow a user to manually grasp insert 24 at notch 27 as shown in fig1 and 18 the user manually distorts legs 34 of insert 24 in the direction of arrow u201c b u201d this bending is facilitated by bend recess 28 formed in end wall 36 of insert 24 which reduces the bend strength of insert 24 by providing a ready crease in the material as legs 34 of insert 24 are bent inwardly d shaped nub 30 is released from d shaped recess allowing insert 24 to slide out of u shaped channel 23 in the direction of arrow u201c c u201d at bottom end 21 of frame 9 as shown in fig1 inserts 24 a are manually pulled out of u shaped channels 23 proximate bottom end 21 of frame 9 in the direction of arrow u201c c u201d inserts 24 a are fully extracted and may be replaced if worn or damaged as shown in fig2 door 25 is then manually lowered to the closed position by moving door 25 in the direction of arrow u201c d u201d the process of removing inserts 24 a is then repeated for inserts 24 b door 25 is now less restrained within u shaped channels 23 because the space occupied by inserts 24 is empty as shown in fig2 to remove door 25 it is manually moved in the direction of arrow u201c e u201d this exposes sidewall 22 b as it retracts from u shaped channel 23 as shown in fig2 sidewall 22 b is free to move in the direction of arrow.