Advice for making narrow bore low D whistle from hardwood

Had some nice hardwood fall from a tree during a windstorm, and thought I could make a low D whistle from it. Having some difficulty with voicing for the narrow bore – just not sure how to diagnose. I’m getting a somewhat breathy C# an octave higher than a low whistle (and a barely audible lower octave with very slight breath). Just curious if there might be any advice on making a narrow bore low D whistle. If all else fails, I can probably chop it down to make an alto G with more forgiving geometry. Just ordered a copy of Low Whistle Makers Anthology and have been looking through previous posts, but any help is appreciated!