In about 1993, somebody I would like to know who had the idea that if you were to ferment bread or beer, that the yeast could be used as a cheap source of CO2. Different recipes have been tried, and most work pretty good. Most people use an empty plastic two liter soda bottle. You drill, or melt with a hot nail, a hole in the cap to accept an air line. The most common problem that I have heard of is that the CO2 gas escapes from this hole in the cap. A sure fire method of sealing the cap, I can say, because it is what I have used is to seal a short piece of rigid tube in the hole with the brand name glue Goop. The Consumers Union Magazine rated glues a few years back, and found that Goop was the only glue to reliably stick to polyethylene used in the bottle caps, etc.. Other people have also reported success using aquarium silicon and also with the use of a fitting sold for drip irrigation. In any case the seal must be air tight.

Website: http://www.hallman.org/plant/bhco2.html