Below are various styles of shelf pin designs. The diameter of the pins is 6 mm, which can perfectly fit the pin holes of all kinds of standard shelves.

Personally, I don't really like plastic shelf pins and prefer to use metal pins, but these designs are still quite practical. To ensure strength, I recommend setting the print infill density to 100%.

This model was created by Tinkercad, and you can edit and view it online:
https://www.tinkercad.com/things/fUVnr5pTtjL