I just wanted a pen holder to fine tune and adjust the machine and would probably not use it again. Most of the designs I see are good but complex, big, require springs and force me to remove the router. I designed a simple one for my Primo with a Dewalt 611 but you can adapt it for other machines and setups. It is held on to the Dewalt mounts with a small C clamp.
It uses a stiff springly live hinge to cushion the Sharpie and clamps to the Dewalt holder. I embeded a number 4 insert in the side to hold the pen with a thin bolt. An M3 would work also. It is not as stiff as other designs but works great if you are careful to just touch the paper just enough to leave a line. Just put a scrap of paper under it and move it around as you lower it the final .1 mm increments. This simple, crude design drew a perfect 100 mm square so it is good enough for its purpose.