air-wound inductor Q

What's nasty about it? There's more going on than you fell happy coping wit h, but difficult and demanding aren't - of themselves - nasty.

In the frequency range you are interested in a nickel-zinc ferrite would wo rk, and would confine the stray fields.

You have to find out how bad the hysteresis in the actual ferrite was - whi ch is in the data sheets - and use the John Chan hysteresis model in LTSpic e to find out how much harmonic distortion you'd get, which would be even m ore work of the complicated design kind that you try to avoid.

Presumably your atrophied work habits aren't up to tackling anything that d ifficult. Mine haven't atrophied down to that dismal level, but I'd want mo ney to do it for you ...