i have an opening in my wall for an air conditioner. i had to recently replace the air conditioner and i can't find an air conditioner the size of the old one. i bought a new a/c and its smaller on each side by about 4 inches and top and bottom by 3 inches. i would like to know what the best method would be to make the opening smaller. the a/c also sits inside a metal cabinet that can't be removed.