Instead of a lever and bolts which makes changing pens difficult I have built a tube based mechanism. The tube holds the pen vertically and it can move up and down freely. The servo moves up and down and so pulls up and down a piece of very thick wire wrapped around the pen.
What is not shown in these photos is the bolt I put down the top of the pen to vary the pressure on the paper.
I had to use old bits of metal I had lying around to fix the tube and servo in place.
The removal of the pen is a doddle with this system, just lift it out!
Another problem is that the pen wiggles about to much in the tube. I’ve now doubled the length of the tube which reduces the wobble, but I need to find something to wrap around the pen so that there is less space between the outer cicumference of the pen and the inner circumference of the tube.