My hands are a very different shape: >4" across the palm but much the same finger lengths. Therefore I have (almost) the same problems in span ... and I can (reasonably) easily finger my low "D" Overton with a "half piper" hold - that is, with my left hand straight but my right hand pivotted from the ring finger to account for that very distant bottom hole.
I guess the bonus I have here is that the pads of my fingers are wide and I have no great problem sealing the holes. Narrower fingers mean that placement must be more careful ... or just plain impossible if the hole is wider than the finger pad.
I suspect that the low "D" Overton might be a problem with your narrower fingers, but it is possible to position and size the holes differently (with some theoretical losses in chromatic playing - but that is not really the 'folk' style). The real problem is the lowest hole and I am sure it can be placed closer (and the second hole made more even). A custom made whistle could also offset that hole a little to make it easier to reach without any compromise in tuning.
If you must buy a production whistle by mail, I suggest a lot of effort to lay hands on other people's whistles of different types before you lay out the cash.