O-ring size...... what I did is got an O-ring that was about the same outside diameter as the plates, and a bit thicker than the dimpled part of the plates.
I found that I had two in my stash; a red colored one and a black one. The red one was softer material and when the brake lever travel was adjusted , was too soft and the brake light came on just moving the handle bars. The black one, a bit harder and worked ok.