How about making a square exactly the size of the narrow side of the rectangle? Draw a pencil line diagonally from one corner to the other. Place the square right sides together over the end of the rectangle matching the cut edges. Sew on the diagonal line and trim 1/4 inch from the stitching line. Now the rectangle should have a half-square triangle that finishes off the rectangle! Hope that is what you are wanting. Happy quilting! :)