I was watching program where this guy put in a square hole in concrete wall for duct work. He went at it all wrong. What you do is cut the bottom all the way through and then put welding rod 1/8" in the slot. Once that is done then you can cut both sides and then the top. Once the top is cut loose it will drop down on the rods and then you can take a couple screw drivers and inch it out. Since this piece will be very heavy you will want to put some 4 x 4's on the other side for it to fall onto. I have done this several times. If the saw that you are using doesn't cut all the way through you will need to drill a hole all the way through where the corners are and then cut the other side from hole to hole. Again starting with the bottom only before cutting the sides and top.
