Is this pretty? No. Is this practical? Maybe. Is it reliable? Yes, and it's all it needs to be.

Assembly

Prints in 30 minutes as a barrel and clip, which then snap together. Optionally, a small screw can be run through the top to hold the two parts together. However, I've found friction alone does a plenty good job.

The cap is as simple as pushing the two parts (body & clip) together, & then snapping it onto the end of the pen. Again, a screw can optionally be used for extra grip.

Cartridge

This was designed around the Uniball Zento UBR-Z-05 if that provides a good reference. I get mine from Oomomo, but any (good) Japanese dollar store should carry similar carts. The tolerances are generous, so as long as it's the generally right size & shape it'll do. 

Known usable cartridges

• Uniball Zento UBR-Z-05

• Zebra JF 0.5 JAPAN 2411 (I currently use these)

Notes

• Loosely inspired by the Sakura ID04 pen with my own personal touches! (One of my personal favourites)

• Some printers may struggle with the tip of the pen, but a few supports should do. Nothing substantial!

• Need a quick Valentine's gift? There's a version with a heart for that! 

• The cap's clip and the main pen's clip are different! The cap extends itself farther from the main body - Easiest way to check is seeing if the clip naturally rests in the indent on the part. If it's getting forced out or doesn't sit in, it's probably the wrong part!

• Don't want a pen clip? Use the barrel plug version! (DSP-ID03-PLUG.stl) Also recommended if you're using a cap for the pen.

• The cap can also be used without the clip or plug if you just want something simple!

• But yes, this is my techno-edgelord gel-pen.

This file is provided as-is!

I designed and tested this in a single afternoon, with various follow-ups in the 48 hours after that. I do not claim for this to be perfect - However, it's satisfactory to me, and good enough to publish.