A hand-crank wheel, like this one from Summit Racing, is a lower cost alternative, but requires repeated trial and error test fitting and measuring to sneak up on the desired fit. Some builders will file from only one side to manually align the squareness of the cut. Always cut with the wheel turning inward towards the center of the ring, as an outward cut will flake the moly from the ring face around the gap. My favored technique to control the parallelism is to press the ring firmly into the stop pins if more material needs removal from the outside of the gap. Alternatively, to take more from the inside of the gap, the ring is pressed lightly against the stop pins and squeezed more tightly in the center. You have to develop the feel.