Add a knob widget

Hi,

I'd like to have a knob widget.
My use case is to display some selected fx params, some are ok for sliders, but in some cases I would prefer some simple knobs.
To give a bit of background, I'm writing a script to use Reaper as a pedalboard for my guitar effects / amp sims.

Maybe it's possible to draw it directly in a custom widget, but I don't know how.

Any help appreciated.