I got sick of my nib holders plastic degrading and not holding nibs that I made my own. I printed the insert in tpu, but petg or pla might work too. The smaller one is meant to replace existing holders, and the two with threaded parts are a fully new pen.

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Print high resolution, use a brim and, hopefully it works. I only printed the actual holding piece in tpu, so that would be my recommendation. If you have adhesion issues, or it's too loose, I also have a tighter holder.