As a custom callmaker, I can tell you the "best" way to cut the lip is on a lathe. You will have a hard time doing it consistently any other way. It can be done with Forstner bits, but then you have to punch the guts out of the pot and leave a pedestal in it, unless you are going to glue something like a washer or shiminside for the soundboard to sit on. You can use a 4-1/8" hole saw with the pilot bit removed to cut the pot out, but you have to be careful that it doesn't twist or shimmy on you. The most common way is to scribe out a circle with a compass and cut it out with a bandsaw. Then you put it on the lathe. I know some have used a router to do the work, but it is a lot more work to get consistent from call to call. If you are just trying to make one for yourself, this may work. I wouldn't do it for multiple calls. A turkey call is a musical instrument. Every little dimension will affect some aspect of the sound...sidewall thickness, bottom thickness, call depth, pedestal height, etc. all have an effect on sound.