A D whistle with an added hole for C below D is not a new idea, but I had not pursued it seriously before now, to add it to the range of what I can offer. The new D Plus whistle has a wider bore, same as the standard C whistle. It plays the same as a D whistle, if the extra hole is not used. But you have a number of extra possibilities: rolls and crans on the bottom D, an extended scale, playing pipe tunes which use the flat seventh below the base note, playing in C major (by half-holing R2 for F nat), using another alternative for C nat, half-holing the R4 hole for C#, and probably more.
I also experimented with an extra top hole, for the left hand thumb, to give an octave D. Apart from that by half-holing this one can play the top second octave notes softer than otherwise possible, so they blend better with the first octave. This technique is used on recorders, and I think it is essential on those instruments, but commonly not employed on whistles. I thought it is a useful trick to add, and on the whistle it is not essential, but an option. I call this the D Plus Two whistle, because it has two extra holes, eight in all.
This is the same number of holes as seen on recorders. Apart from the tone and playing characteristics (and all the different geometries), a recorder with baroque fingering allows for forked fingerings for F natural and F sharp. The D Plus Two whistle does not, F# is played as on a normal D whistle, and F natural must be half-holed. I have been experimenting in developing a forked fingering solution as used on a baroque recorder for the whistle, and I got quite close. But with a cylindrical body I could not quite make the F# low enough, it stayed a bit sharp. Or, alternatively, the G was a bit too flat. Getting G, F# and F tuned just right eluded me so far, even though I have come very close to an acceptable solution.
So I decided for now to stay with the new D+2 whistle model, have a standard F# and no forked F natural, but use half-holing instead for this note. In time I might arrive at a fully forked F# and Fnat whistle, but I guess I need to do something with the bottom bore, in other words: it will get more complicated! Any ideas on that would be most welcome! I think there could be a bridge from whistle world to recorder world in form of an eight hole whistle.
… either way, from practical experience (I’ve made several 8-hole whistles with greater or lesser degrees of success), keeping the finger-holes slightly (or even significantly) smaller than “normal” makes for an instrument that cross-fingers much more readily than one with “full-size” finger-holes. Inevitably this tends to make for a quieter whistle, but one with greater flexibility. I’ve very limited resources as far as off-the-shelf tubing is concerned, which has doubtless limited my scope, but I feel the concept is “do-able”, given an appropriate choice of bore-size to key. Consider how much smaller the finger-holes on a descant recorder are in comparasion to a C or D whistle and this might give you a few clues 