How to have certain files in certain paths automatically after installation of the Debian (server)?

I have been requested to make an installation ISO for the Debian server such that: 1. Certain packages need to be pre-installed on it. 2. Certain files (mostly Python modules) need to be located in certain directories automatically. ISOs are given to customers and these conditions are there for removing the need for any other actions by them after the installation. Currently, I have achieved the first condition by using Simple-CDD ; however, for the second one, I don't know how I can achieve it. (One thing that is mentioned in the Simple-CDD HOWTO page is that files needed could be listed in the profiles/NAME.extra *so that they will be added to the /simple-cdd in the ISO file*, but I need those files in the filesystem of the Debian itself after it is installed).