This is perhaps not possible, but here's hoping someone else has tried with more success than I have.
My standalones have needed to span all the way back to Tiger.
Official OS recipes for making icons switched to iconutil starting in Sierra, however none of those work with Tiger - standalones just show the generic app icon.
For a long while I've been using an El Capitan machine to make the icns file with an apple-suppled app called "Icon Composer" which was discontinued after ElCap.
This seems to work Tiger through Mojave, however only has 16px-512px art. None of the retina-display 2x items.
The high-resolution stuff is not that big a deal, but here's hoping someone has made a discovery.
Thanks,
Greg