Creating a private repository is very easy. For example, using GitHub:
1. Create a private repository (https://docs.github.com/en/repositories/creating-and-managing-repositories/creating-a-new-repository) and import the SSH keys in your machine/server/docker...
2. Upload the phpdocx namespace package to this private repository adding a custom composer.json file. As explained on Integrate phpdocx with Composer projects composer.json must use classmap to autoload phpdocx classes:
3. Update the composer.json of your project adding the private repository and install it as any other package/repository.
These steps are the same for other Git repositories such as Bitbucket or GitLabs.Please note that this is a Composer question (Using private repositories with Composer) and you can find detailed instructions on many pages. For example:
Learn How to create and publish a composer php package: https://dudi.dev/create-composer-php-package
Using Composer With Private Packages Hosted On Github: https://dudi.dev/composer-private-packages-github-repository