Here are different designs of shelves pins. The diameter of the pins are 6mm and should fit all standard shelves pin holes.

I myself don't like plastic shelf pins and prefer to use metal ones, but these can be useful anyway. I would print it at 100% infill for strength.

This object was made in Tinkercad. Edit it online
https://www.tinkercad.com/things/fUVnr5pTtjL