I was really fascinated by Azetec death whistles, and decided to reverse engineer one, by using cross section images on Google, and explanatory videos on YouTube.
The angle and strength that you blow with into the whistle affects the sound (subtly).
This is one of the better sounding ones after quite a fair bit of trial-and-error. What do you think?
Shell thickness: 0.39mm (or less)
You can scale it up to decrease the fundamental frequency, but it will require more effort to blow.
Snip off any stringing (if any) especially around the whistle holes. They make the sound duller.
The whistle can get really loud! Remember to protect your hearing and play with it responsibly.