It's pretty small, and I did play around with different sizes of windway before settling on this as the best, but I think next time I will try wide-and-thin like on our modern Irish whistles (that said, the only reproduction I've seen the plug end on also had a small hole on the squarer side of rectangle like this) and maybe a smaller version of this rectangle just to check. The window is definitely larger than most of the other ones I've seen though.
The historical ones don't (I assume for ease of construction) have shaped fipples, but it's something I will absolutely do on more developed ones (unless I'm making an archaeological replica, which I probably will with some). It's not just more comfortable, the jaw position requires is simply not as good for blowing (though that may be partly inexperience with this kind, I think it's partly body-mechanical as well)
![Image](https://i.imgur.com/qEgDejA.jpeg%5D)
![Image](https://i.imgur.com/ODzDF75.jpeg)