Unremovable watermarks in excel

Morning fellas,

From time to time I send out excel models to clients. As much of my information and reports are based on these models, it is always a bit risky to give away proprietary files. In order to not be exploited, I want to add a permanent watermark to the models prior to sending them out stating "Model built by XX for YY". Is this possible?

Described differently, I am thinking it could be done by be putting a watermark in, call it, a masterfile-model and then making a copy where the "slidemaster" can't be edited.

Many thanks for any help!