To produce icon.icns for mac I used:
https://iconverticons.com/online/
I uploaded icon.png and then downloaded the PNG zip on far right.
Then I unzipped and used tools described here:
https://dentrassi.de/2014/02/25/creating-mac-os-x-icons-icns-on-linux/
...so basically:
png2icns icon.icns *.png
But it barfed on a few resolutions so I just removed them and tried again.