How to make derived widget and add into Widget Box in Qt Designer?

Hi everyone,

I am trying to set up two different fonts for a QPushButton, however, the existing one only support one font.
I would like to make a RichPushButton derived from QPushButton to support different fonts, and add to the Widget Box in Qt Designer for the UI/*.ui design for my own module in Slicer.

Assume that I wrote a Python script for class RichPushButton, how can I add it to the Slicer Qt Designer?

Thank you so much in advance!