the original pen module didn't support my 0.7mm uni posca so I created the inner piece of the module with an 11mm hole so that the uni posca would fit in perfectly. in the model I put 4 small holes for those who want to secure the pen with small screws, however I also left the option to secure it with a rubber band(and it works very well!).
The model as it is makes a lot of play with the other pieces and the arm can't draw very well. that's why I scaled up the model by 5% in the slicer only it took some good coats of sandpaper. you might try if with 3% or 4% you can avoid sandpaper.
The hole is 11mm feel free to enlarge and decrease it as you like.
To mount the printed piece, simply unscrew the top three screws of the module and remove the inner piece

Creality

Doesn't Matter

0.2mm