Chris, it isn't that critical...at all. Line it with 1/16 balsa first. Don't worry about strength, that hole could be 4 times that big and still fine.

Next, put 1/4" x1/8" lite ply or basswood or whatever rails in, and check again. On any standard sized servo I have ever used, it's perfect. On some, it's TOO small.

This gives plenty of clearance for the wire exit from the servo without chaffing. if you've had one chafe and go out before, you'll ALWAYS give plenty of clearance in the future. This is cut with the same template that I use on my own wings. it's not an accident.

-Mike