Make yourself a false baseplate such as the attached picture, this will be screwed to the base of your router, then with a pin through into timber turn and cut.
Should you want to make without a centre pinhole, then still make a template this way and then stick to another piece and rout with a pattern bit with a bearing guide, you can then make as many as you wish.
An even simpler one can be found if you scroll through my gallery.
A friend is a good egg even if they are slightly cracked! Derek