I am building a CNC machine and needed a pen holder to do some testing. This is the result, it worked very well and is so easy. Its designed to work with the new Sharpie fine point pen, but you could make it work with most any pen that has a round barrel. In the picture you can see its plotting out a PCB board.
To make this holder you don't need much some acrylic, 2 o-rings that fit around your pen barrel, and a spring. Use the two o-rings to control the amount of movement of the pen. The spring can be what ever you have laying around you do not need much pressure, so don't get carried away. Glue the parts together with some acrylic glue, let it dry and try it out.