I needed to make a round hole in a board and best way was to use a router. The radius was 81 mm, but I had no trammel or something similar. So I used the material at hand and added a small piece of wood with a spike in it to the guide stop.
The small piece of wood fits in the channel that slides over the guide rail. Two end stops prevent it to slide out. In the middle is a small screw, sticking out 2 or 3 mm. I had to file off a little of the head to let it fit into the guide stop.
Before routing you can adjust the guide stop to the correct radius. Then place the piece of wood with the spike on the centre of the circle. Place the router with the guide stop over the piece of wood. Adjust the depth and start the machine. In my case I left the last mm of the trench uncut to keep the center intact. With a contour bit I cleaned up the hole.
Compared to the super jigs and additions of other members, this is a very humble addition.