I remember getting planet to put a CO2 valve on a 1.9 air bottle for a shoebox, you could get about 40oz in them! I found a place that does 8ci air bottles recently (steel ones) and was thinking about putting a CO2 valve on to have a bottle about the size of the old 3.5oz ones with a PI mark on it but was struggling to find out if a bottle tested for air is deemed safe for CO2 or if there are different test, I imagine the safety test is just a pressure one so it should be safe since CO2 only peaks at 1800psi? and the bottle will be tested to 3000psi.