Just found these pics of Seattle_Mike 's car. Hopefully he'll see this and post up whether these are off the shelf or custom. Just pull the pin and the bottle comes right out but is totally secure otherwise.
Mike, I have a single 15lb bolted to the stock floor pan with a single brace bolted to the top of the bracket then welded to the cage. I will text you a pic in a few minutes. When I had a passenger seat, it was bolted to the seat bottom with a brace that bolted to the top of the bracket then to the seat back.