A glass bottle made for carbonated beverages would work ok, but don't use just any bottle. Yeast CO2 generators can generate a pretty high pressure, enough to shatter some glass bottles. A champaign bottle would be perfect. An air stone is a very poor diffuser, because the pores are too big. Much better, and dirt cheap is a piece of bamboo chopstick, stuck in the end of the CO2 line. Or, some of the wood "airstones" are also good. If you have a powerhead in your tank, just sticking the CO2 line into the inlet to the powerhead works very well, but is noisy.