You can get away with an imperfect cut, the smoother the cut the better so cut with a hacksaw and file smoother. What holds the stem isn't the action between the wedge and stem but the way that action pulls the wedge into the steerer and not having perfectly the same won't matter much. Like any stem check it every few months to make sure its still tight, they can loosen no matter what. The big issue is the bolt, unless you can thread it my experience is that the bolt is sized to the stem and the threads are only as long as they need to be. If you have a good die it isn't the worst thing to thread but it is one more detail to have to work out.