Ask HN: How do you keep personal modifications to a public repo private?

When I work on a someone else's public repo I might add a little extra scaffolding around it like:

* Scripts to install/configure any prerequisites for my distro * Scripts to launch a VM/container to work inside * Editor/IDE config files to setup the project's linting/testing etc

It's not useful to anyone else, it should stay on my machine. But I:

* Don't want to change the repos .gitignore * Still want to use source control on these files

Are there any tools out there to help? How do you solve this?