I make whistles from polypropylene plumbing tube. Not commercially - I’m not rigorous enough for that - but I have made whistles for friends, and I have prized specimens that turned out especially well. I follow Guido Gonzato’s instructions for plastic whistles (with wooden fipples).
Lately I’ve been working with Overtone flutes, including some home-made konchovkas. Konchovkas have one hole - the windway hole - and it is round. It’s round so that you can blow the overtones more easily. I’ve tried making a konchovka with a square hole like a whistle, and it is a dead loss.
The last konchovka I made, I cut too short. So I thought, I’ll make it into a low D. Not a problem. I had a spare fipple, and cut the finger holes without too much problem. But the rounded windway made me wonder.
The Low D plays fine. I like it. It doesn’t seem any easier to reach the higher notes than on the Low D with a square windway hole. I was afraid the notes might be less clear on the rounded-hole model, but it is not so.
So can anyone give me a whistle-maker’s reason why whistle windways are never round?