There is no real requirement to mount the spring loaded pen or pencil into the collet and worry about the router/spindle rpm or mechanical rotation. I made one for one of my early machine builds that was a brass tube with a spring loaded BIC ballpoint pen inside. The tube was mounted to a scrap wood block that was C-clamped to the router mount in front of the router. x-y zero the pen at the x-y starting point and set the pen a little below the surface to pre-load the pen and it worked fine. Keep the length of the protruding pen/tube reasonably short so that it doesn't flex and you will get accurate drawings. Slop in the pen can be due to too loose of a fit inside the tube. Too snug and it will not follow any minor unevenness in the surface. Be aware that if the drawing is too close to one of your travel limits it may crash if you don't have limit switches. The router/spindle collet is offset from the pen location by a few inches and as long as that is acceptable, there won't be a crash.