If you have a router...
1. Make a circle jig out of scrap wood that will allow you to cut a circle slightly larger than your router base. The size of the circle should be: half the size of your router base plus half the difference between the 12mm hole size and router bit size.
Equation: Circle Radius = ( Router Base Radius ) + ( Hole Radius - Cutter Radius )
2. Once you have this jig, you can cut a circle out of scrap to make a template for your router to ride inside.
Seems like a lot of work, but should provide a challenge.
