Print the body and cap at 0.12mm layer height and the section at 0.08mm

The nib units are Bock 250 or #6, and they can thread right into the section but depending on the printer calibration you might need a bit of effort, if that doesn't work you can scale all the parts by a little bit and that makes it easier to fit

This was my first design, it works, but I have newer better ones :)