Cloning is not only the most secure solution, it is the vendor solution specifically designed to move configuration and data between instances.

Any alternatives for moving data between instances would be less secure as they must transverse external infrastructure.
Copying between instances also introduces additional risk as the target instance must be manually configured to mirror the sources configuration and data structure exactly.

If you are supporting a ServiceNow implementation for an irrational customer who will not listen to basic common sense then I would suggest finding another client ASAP, as it is your organizations credibility that will be at risk if you allow the client to dictate inappropriate actions and it goes wrong.

Perhaps if you provide a time estimate

< 5 minutes to set up an OOB clone.
< 1 hour to set up a clone, if you need to exclude data on specific tables.
100 hours + to configure and test an alternative solution that is less secure and may not work.