Here are good-playing Low D's from several makers
![Image](https://i.imgur.com/uyAbA2u.jpg)
What does a comparison of the finger-hole placement and sizing tell us?
That in truth there's only one way to skin the cat.
Ideal performance dictates the holes have even (or nearly even) sizing, with every note fully vented (for example, a full-sized open F natural hole between the E hole and the F# hole) which would be impossible to finger. This conundrum led Boehm to create his fully keyed flute with evenly sized and spaced huge holes that no-one could finger if keyless.
Human anatomy dictates smallish holes spaced as the fingers are spaced. This leads to an instrument with numerous neutral intervals rather than any Western musical scale.
Low D Whistle makers are forced to compromise between these conflicting dictates, and trial and error have inevitably led them all to the same conclusion.
Make the holes smaller and closer together and the evenness of the voicing suffers- it cannot be avoided. It just depends on how poor a voicing is acceptable.
(The one solution is a conical bore, which brings the lower-hand holes closer together.)