I have a whistle in my collection, given to me by the maker in 1979, formed from sheet aluminum, with a rectangular cross section and tapered like a Clarke original. It has a red plastic plug and is fairly in tune and on the quiet side. He heard me playing a Generation in my car while parked on a street in San Francisco. He was going to sell them, but I never heard anymore about them. My camera is broken, but will post a photo when I get a new one.
I have made Square Whistle (I’m the maker of the one called the Town Square (yes, the name is a pun). It plays just as well was any round whistles.
After having some C&F people here try it out we did find that most had trouble finding the toneholes when not looking. Somehow, round bodies makes it easier to find the holes.
When properly voiced, they have some decent volume - like an organ pipe.