I have a video that shows this on the cnc machine. It's the making of the banana lure video. There is probably others that do it by hand, you cut the perimeter out at the same time with 2 pc of wood then carve them together. Some will use 2 sided tape, some use staples or brads that can be pulled out and filled later, some preroute the inside then put wire/hardware in etc. then glue.
When you glue them use either Gorilla Glue or CA glue. Many exterior rated glues aren't meant for submersion..