You can make a template to fit a router insert plate in a matter of minutes with Templates by numbers.
To make the template we did the following:
1 Make the template plan in two halves (see the seperate article)
2 Piece the two halves together and place on the workpiece
3 Transfer the positions of the 'Mortice jig cutout' guidelines from the template plan. These guidelines have the offset for the router bit/guide bush combination you choose to use when you made the template plan.
4 Align scrap straight edges on the Mortice jig cutout guidelines
5 Start routing out the cutout.