Take a piece of light rail and hold it up under the side of your cabinet. Make sure the light rail and cabinet are flush, and the end of the light rail and the wall are flush. At the corner of the cabinet, mark off the light rail along both the face and side of the cabinet, creating an intersect on the light rail.
Since the two sides of the cabinet are the same length, cut another piece of light rail to the same size, but at an opposite 45-degree angle.
Cut a piece of light rail to a length at least 5 inches longer than the face of the cabinet. Cut a 45-degree angle off of one end. With the angle cut, hold the front piece under the cabinet and flush it up with the affixed side piece. With the two pieces flush, mark off the opposite corner along the front and side of the cabinet. Cut the freshly marked front piece so the 45-degree angle is opposite the first angle and the intersection just marked is the middle of the 45-degree cut.