This is a Dimmer knob - Toggle switch - Rocker Switch Cover Plate. It is an standard oversized plate size. Please let me know if you have any questions.
Painting To Give A Smooth Finish
I painted with 6-7 coats of filler primer and two coats of Gloss Almond paint to get it smooth and shiny. I printed this at 0.28 mm layer height so a tighter resolution would likely require less coats of filler primer.