Easiest approach is the customize-image.sh script in userpatches/. https://docs.armbian.com/Developer-Guide_User-Configurations/#user-provided-image-customization-script

I want to add a private package with C source code in custom image. Extensions maybe are one approach— are there any other more standard practices?

Finally, build successful on Ubuntu Noble.😃

Oh and i missed one thing, the Helios64's board is nano itx (120x120), the on from radxa you want to use as replacement is mini itx (170x170)

Provide logs