Fizik saddle pack clips to the saddle. The large will easily take a 29er inner tube, Hexus II and CO2. I also use a small one and can still get two road tubes, smaller tool and CO2 in that. I use an Innovations microflate – the smallest CO2 device. Obviously you will need a Fizik saddle.
To be honest, given the size of the inner tube, I’d wrap the tool and CO2 inside the tube and secure with a slice of old inner tube, and velcro under the saddle. It can’t be bigger than a tubular tyre! And for authentic old skool, use a leather toestrap instead of velco.