Template "collars" for a router can easily be used to route a hole or
depression which exactly matches the dimensions of an existing piece.
This is handy if you want an exact match for something like a table
insert or you want to route a large depression into a wood surface. I
have used this approach to make a router table with an insert hole for
the router mounting plate. I have also used it successfully to make
the "wells" found on the base of piperacks.
The general idea is to use the original piece (or a blank piece of
wood of the desired dimension) to create a "frame" which will serve
as a guide for the final routing. Both the creation of the frame and
the final routing use collars fitted to the router base. These are
available in various sizes and a set of collars that fit your router
is usually inexpensively obtained at your local tool store.
Here are my (handwritten) notes on how this is done...