Create missing folders
At the moment only the folder for preview images and for content elements are shown as "missing folders" in the Mask backend module.
It would be better to show all folders (Backend folder, mask.json folder and Layouts+Partials for BE+FE,...).
Change default folders in the extension settings
Change all paths from /fileadmin/... to /typo3conf/ext/mask_project/...
The extensionkey mask_project ist reserved by the Mask team already.
Suggestion for a structure inside the extension:
Save a .htaccess file to deny read permissions at: /Resources/Private/.htaccess AND /Configuration/Mask/.htaccess
Discussion 1: Use '/Resources/Private/Mask/' or better '/Resources/Private/Mask/Templates/' (also for the backend path)?
Discussion 2: If the file ext_emconf.php in the extension-root folder doesn't exist, create it and if possible install the extension automatically. Then the paths in the Extension settings can start with 'EXT:mask_project/...'