Need advice on replacing Theremin for interactive sculpture

it‘s a paia theremax. pretty sure i am.

Ah, Yes! It is. I put my phone deep inside and caught a glimpse of 505, with "rem" under it so that's probably part of
9505c
Theremin

And now that you mention it, I'm sure that's what the guy who built them said they were. Thanks so much!!

You might try using non-shielded wire going to the antenna.  Shielded wire is one long physical capacitor which could be hurting your sensitivity / adding to environmental drift. 

Are these sculptures non-conductive?  If so, I'd use a plate inside the sculpture instead of rod antennas (though you could leave the antennas on there for appearances).  A plate would give you more range, and if it were inside you wouldn't have to worry about ESD zapping the electronics.  If the electronics can be tuned to it, the ideal plate area would be roughly sized for the thing you are sensing (for a human hand maybe the size of a license plate, for human bodies obviously much larger).

In the end though, you are tuning for zero pitch way out, which is always going to be a touchy adjustment regardless.  The D-Lev software zeros out the negative pitch field, which would help, and the calibration is dead simple.  One could probably modify the software to do a long-term auto calibration of some sort, to counter drift.  The synth + voice presets could make it a lot more fun to interact with (bells, helicopters, planes, human voices, wind, wild FM, etc.).

This looks like a job for Superman!