By the way, if anyone wants to design and 3D print their own phase plus, the phase plug on the Celestion CDX1-1425 is plastic and removable. So a great candidate for tinkering with this.
Celestion CDX1-1425 Neo 1" Compression Horn Driver 25W
I'm not thrilled with the price - at $100 there's quite a few choices that are superior. But a good candidate for phase plug tinkering.