Or else some way to clamp it down REALLY securely. Depending on the hole size and where it is on the side of the round stock, I'd mill a small flat with an end mill, use a center drill to start the hole, then drill to size. If it's closer to the center line, you might be able to get away with skipping milling the flat. I'd say the trickiest part will be getting the hole location precisely located. But at $85-100 for a "factory" replacement, you can mess up a LOT of round stock and still come out way ahead
BTW, I don't remember what you have for a mill, but if mine is bigger/heavier/more rigid and I can help out, let me know. Later.