CNC 1610 pen holder

I tried another pen holder for my CNC, but I found that it didn't have any support near the pen tip, and the style of pens I'm using (Staple's ultra fine tip permanent markers) really don't draw well without support. I've started using CEM-1 boards for isolation milling, and the white back is perfect for drawing the silk screen with a pen, hence I needed to get this working reliably.

The holder fits nicely in the laser mounting grooves of the 1610 carriage.

The holder is parametric, so it