The note can be found attached to the dome at ATLAS Observatory. To access it, go up the wooden staircases and follow the walkway facing northeast.
I'm thinking about a big symbol, right on the front of the dome. Put a spotlight up there, aimed at it, so folks can see it at night. Really let everybody know the Brotherhood is here.
Have to do it myself, I think. I wouldn't trust the 'bots on these walkways. Maybe some kind of pulley system to lift it?