I've seem posts where people mount a board inside the cabinet, centered over the hole to be modified, and then a Jasper Jig/ plunge router como is attached to the board. Not perfect, since there is some distance (1/2" - 3/4" due to baffle thickness) between the center point of the router's arc and the actual cutting surface.Only time I've ever used a keyhole saw if for cutting drywall. That must have been a chore cutting wood ......C