This fipple is inspired by mikaelfernstrom's fipple. I decided to make this customizable, in hopes that people would experiment and, e.g., come up with the best combination for making a loud, clear whistle. You can also download the OpenSCAD file and use it as the mouthpiece of an ocarina or tin whistle.

I printed this one, covered the end, and blew it next to a guitar tuner; it claimed the whistle was a (pretty shrill) B. I compared that with the rough estimate for a Helmholtz oscillator, and that seems about right. There's a calculator for such oscillators here: http://hyperphysics.phy-astr.gsu.edu/hbase/waves/cavity.html#c4