I built a Theremax and have modified it for one handed playing. Moving right and left for pitch and up and down for volume. It is much more intuitive for me this way, but I'm not a theremin player so what do I know... The main mods are with the antennas. Two Lev antennas with a slight amount of tweaking (thanks Christopher!). I didn't get along with the stock antennas and they wouldn't work as a one handed theremin. I tried!
Grasshopper
Grasshopper