I am using https://github.com/drupal-composer/drupal-project . How to add more scripts after "create-project"? I want to use https://github.com/drupal-composer/drupal-project to generate several customized projects.

If I want a contrib module to be available for all instances, what is the best way to get them when running "composer create-project"?

I thought to add this logic (to add contrib modules by default) in scripts/composer/ScriptHandler.php but this file doesn't seem to be suitable for customizations.

Is there an easy way to add other scripts to run on project creation?
Or which other way should I chose?
when you use create-project you're creating a new composer project with a  composer.json based on the package type.

The composer.json file is highly customizable. You can set install paths for specific modules/libraries and/or set a list of default modules to install when the project is created. You can also apply patches and run post-install scripts if nessecary. Plenty more options as well. I suggest jumping into the composer docs.

to add a module to composer.json run:

composer require drupal/module_name 8.1.*@dev 


Once you have a composer.json the way you like, you can take that composer.json file, place in a different empty project directory and you can simply run composer-install and it will install the drupal project, all the modules in the appropriate directories specified inside the composer.json file, post install commands ect, it will also write a composer.lock file if one doesn't exist. No need to create a brand new project everytime, just manage custom composer.json files thats the beautfy of composer. 

https://www.lullabot.com/articles/goodbye-drush-make-hello-composer

https://getcomposer.org/doc/01-basic-usage.md

https://getcomposer.org/doc/