Pen body with integrated spring

Very useful for pen-plotters etc to get an even force onto the paper for ball-point pens.

* Take the ink cartridge out of a click in/out type pen
* Move the spring to the top side of the, wide part up
* Print one of each .STL
* Insert ink cartridge with spring going into top
* Press the two parts together, and tape them

Designed for 3mm diameter ink cartridges, outer diameter of body is 14mm. Both are parametric and change-able in the attached FreeCAD source files.
The depth of the sp