It all depends on what type of side rail you are using.
I used a
1-1/2" piece of wood which I used to rout the arc into. I made a matching mirrored rail for the other side they are 6" apart because my router sits in between the rails keeping it steady as I run along the rout path.
What you could do is make the arc pieces first then add outside pieces to get the same effect.
Or...
Forget the outside pieces and do what Jwell does with his guide. Both will work.
Bill