Create pure co2 environment

This may sound strange but I'm working on a alternative health treatment using carbon dioxide, where the subject sits in pure carbon dioxide. I've tried just filling a flattened out garbage bag with co2 and then enter into the bag from the top. I assume that the co2 stays in there because it's heavier than air. However I'm worried about leaking out. Ideally I was thinking of using some type of container (a plastic garbage can for instance) where you pump co2 in from the bottom and wondering if the regular air would be pushed out if we put a small whole on top of the lid. Theoretically the participant could go in entirely wearing goggles and a snorkel that came out the top. CO2 helps with things such as for the skin as well as for dialating arteries as well as many other things. My task for you would be to help me figure out a way to be immersed in a pure co2 environment. I have a 20 lb tank of compressed co2. Thanks!