In fact, if you want to use something like Nix on a UniversalBlue system, you have to spin your own. The "hotfix" and chattr solutions of pre-composefs don't work anymore. Anything that needs to go into a read only location and isn't package as an RPM requires you to "spin your own".

Luckily UniversalBlue makes it incredibly easy, they have a template repo you can use that has all the GitHub action setup included to auto-bills on every change, and directions for how to set it up. It took me about 10 minutes