Bag breathing seems like a huge chore for me, never really find the time to sit down and have a good 10 to 15 minute bag breathing session. When I have on the few occasions, I've always been disappointed with the results. I have a soda stream machine that comes with co2 tanks, technically it's suppose to be used to make carbonated water. Could you possibly blow some co2 (from the tank) into a bag or balloon and breath it in? It would seem like a more concentrated dose of co2, rather then re-breathing through a bag.