This model is based on an normal version (2 wall layers) of a Tactical Ballpoint Pen https://www.printables.com/model/412788-tactical-ballpoint-pen. I adjusted it for use with Pilot G2 refill. I also added a small protrusion, to which you can attach a clip from a piece of wire.
I printed it with 0.2mm layer height, with organic support inside the pen (see the 3mf files). Print any material you would like. I printed it with PETG.
Print one pen and one endpiece. You'll additionally need an M3x10mm screw for the handle to screw in the side of the end piece. Set it together with a Pilot G2 refill and spring. I used slightly wider spring (6 mm diameter, cca 16 mm length).
For more precise writing control, you can heat the tip of the pen and adjust the diameter of the hole around the cartridge for a tighter fit.
The author remixed this model.
Adjusted for Pilot G2 refill, discarded clip, left just a tiny piece to insert heated wire (e.g. paperclip). Also printed from PETG, instead of PLA, so it theoretically should be more durable.