How to make a template for routing (with guide bush)

[ToolbeltDandy]

I'm helping a friend with a fairly basic woodworking operation. I need to inset a round metal doorbell push button into a piece of wood and I'm going to use a plunge router with a straight bit and a guide bushing.

The problem is creating the template. The template needs to be 2.5 mm wider than the intended cut. How can I do this? It's made a bit more awkward by the doorbell not being perfectly round - I'd like the template (jig?) to match this imperfection so that the flaw isn't obvious after insetting it into its wooden panel.

 

[gregers]

can you not clamp the piece of wood and then carefully free hand the router then using a chisel pare off the excess?

 

[Dave54]

In my experience these sort of jobs are better done by hand, marking out the recess from the item itself, unless you have multiples of the job to do. All making a jig does is takes time, and increases the possibility of error.
Do what gregers says. Freehand most of the waste with a router and clean up by hand.