Summary

This was designed for a project where I need to cut loads of holes out of wood with a plunge router.

The idea is it hooks into 2 pieces of m8 threaded rod, which fits into the guide holes in the router.

Instructions

You may need to adjust the parameters and re-render the scad file before printing, the attached STL is for my router, which has 8mm holes that are 84mm apart.
Print
Fix a 608 bearing in the central bearing hole, and fit the central piece through the middle of it.
Attach two pieces of M8 threaded rod through the 8mm Holes, and bolt on both sides.
Insert the threaded rod into the guide holes in the router. the distance between the central hole and the inside of the router bit is the radius of your hole.
Put a screw through the central piece and into the center point of the piece you are cutting, and cut!