The prong of my dad's belt did break. (A prong is that tiny part that prevents the belt from opening up on its own. Search for “belt anatomy”.) I was not too hopeful that I could succeed with a printed part. And less so with PLA. But it turns out that the result is quite sturdy and should hopefully last at least for a while. Printing a new one is about 3 cents and prints in 5 minutes, so repeatedly replacing a replacement part is a viable option, in principle…

The prong has a “hook” that can be squeezed into the buckle's bar. This design's hook has an inner diameter of 2.7mm. The prong has a square profile (2.7x2.7mm) that fits into a typical belt hole (~3.8mm).

The prong probably needs to be adapted for each belt. The FreeCAD source is essential.

PrusaSlicer gives a warning, but this part has actually pretty good adhesion and stays solid on the bed – its small height is the key.