Simple pen holder concept. Back plate to be mounted to Z-axis.
Rubber band at rear maintains positive tip pressure while moving across paper, compensating for tip wear or uneven surfaces. Set tip location/height to work surface similar to the way bit height is set and then lower Z-axis another few millimeters beyond tip contact to maintain positive pressure. Stop bolt at side holds rail in place when axis is lifted.
Pen can be mounted using rubber bands or hose clamps. Mounting system allows for variations in pen diameter along its length. Rail can easily be removed when not in use.
And finally, mounting the assembly on the Openbuilds Router/Spindle Mount. Note the 4mm holes used to access and tighten the (4) M5x8mm mounting screws.