There are limited options for distributing container bound scripts. When a document with a container bound script is copied, the script is also copied. So if the 300 people in your organisation copied the document they would also have a copy of that script. The issues with this techniques is that the script is only attached to that particular document and creating a new document, rather than taking another copy of the template, or opening an existing document will not include your script functionality. The other issue the script in the copies of your template becomes orphaned and you cannot push updated versions. Using Managed Libraries addresses this to a degree but the end user still needs to select the newer version of the script in the script editor.
A way to mass deploy your script in your organisation is using Google Apps Script authored Add-ons. Add-ons are installed by users and become available to every document they work on. The script behind add-ons can also easily be upgraded without any user intervention. One issue with this approach is all add-on's need to be approved by Google. This introduces a risk that Google will not publish your script as an add-on. Whilst there is a public store of add-ons after your project is approved by Google there are visibility options to restrict access and prevent there general visibility. As per add-on publication step #12:
In the "Visibility options" section, select which users will be able
to find your add-on — usually either all users or only users with an
account in your Google Apps domain.
Hopefully when add-ons leave developer preview Google Apps domains will have more control over publication and visibility (I've opened a feature request for this)