Tin snips then rip with your hands. You're going to cover the edge with a miter box or with an end cap anyways, right? If you really want to use your miter saw try a reverse plywood blade. meaning install the plywood blade the opposite way the directions tell you so it actually drags along the metal instead of biting into the metal. You'll get a much cleaner cut that way. I think the saw will slow you down though. I can get a real clean cut with tin snips and even ripping the metal by hand.