yes,I have a 56 key tenor treble,although I dont use it quite as much as my 48 key,It being a little heavier on the wrists.its very useful for recording.
I hardly ever go above high c sharp[topstring on fiddle],I find the notes very squeaky,To my mind the idea of building a box, that has the range of the tenor treble without the notes above high d[on top string, fiddle]is quite a good one.Dick Miles.