The best way to make it perfectly even will be on the lathe for sure. Now, the idea of ROUND, you will have to work on some kind of tool that fits you best. I can tell you, my go-to rounding tool for call mouths and all is a Sorby 1/4" round nose scraper. 863H is the model. I am surprised more call makers do not have this little guy in their arsenal. I use it on every barrel and every striker that comes off my lathe. And many more parts than that.