if you play guitar right handed, you'll find playing the theremin right handed easier to use (right hand pitch)
I'm left handed, polay bass and 'horizontal " string instruments that way.
I play cello and accostic bass right handed, don't ask.
I find it is more comfortable for me to play theremin left handed as well.
I can play right handed if needed, but I'm not as accurate.
Build it right handed.
Andy