Slugbaby
I've played a friend's Thermin a couple of times. Fun, but a b**ch to control.
I'd rather fake it on my guitar with a slide and Ebow (slide to bypass the fixed pitch of the guitar, ebow to address the attack and decay).
Having given up fighting a theremin years ago I use a Steve Hillage/Daevid Allen/Syd Barret style glissando technique using a steel bar, a volume pedal, wah and lots of compression/fuzz and modulated delay.
Not quite the same as the theremin because you can’t smoothly glissando across the entire range of notes because of the need to cross strings, while you can gliss through the entire range of a theremin, but much less sanity challenging than the only instrument you play without touching and puts out bum notes if you so much as breath at the worng moment.