Found this one: cubicVR. A free plugin by Sketchup Podium.
Description and download here:
https://www.suplugins.com/podium/help/cubicVR.php

It generates the equirectangular output in one clic.
The drawing lines are pretty sketchy, but it's sufficient for my current needs of fast prototyping.

Though, I would be glad if someone knows of a cleaner alternative.

cubicVR first output

cubicVR equirectangular output.

Equirect output - closeup