The bars on the original MPCNC drag knife / plotter pen adapter had a 100 g/mm spring constant:
Making the bars slightly thicker improved their print-ability:
The reddish tint marks the new bars, with their location carefully tweaked to be coincident with the stock STL.
Shoving the pen into the scale with 0.1 mm steps produces another unnervingly linear plot:
Real plotter pens want about 20 g of force, so this isn’t the holder you’re looking for.
A bunch of plots at Z=-1.0 mm turned out well with the ballpoint pen insert, though:
The globs apparently come from plotting too fast for conditions; reducing the speed to 1500 mm/min works better.