I recommend sulfur hexafluoride for something harder to replicate. Nothing like making hackers risk their life to impersonate you

Or skip the half measures and go straight for the dioxygen diflouride.

https://www.science.org/content/blog-post/things-i-won-t-wor...

Excellent idea!

Do you need to calibrate it to be able to repeat it, and does that calibration change if you are at a different altitude and in different conditions, such as humidity?

Does merely changing altitude (or ambient pressure) change voice enough to be considered different by a recognition or synthesizing system?