i use natural tapered corks. This works just as well as the grolsch/ fischer/ flip top bottles, and it eliminates the need for the plastic/ rubber gasket on this flip top bottle (I know a lot of people are worried about leaching chemicals into the elixir).
Just make sure that they are in there good and tight. If not, what can happen (as this happened to me) the cork can fly off in an explosion of kombucha. If under pressure, do not shake the kombucha at any time, it may cause the cork to fly off. This most likely would not have happened if I were to have used a corking press thingy with regular, non tapered corks. I use tapered corks because I do not need a cork screw to uncork them or any other extra tools. Its very simple. Sometimes I even add a capsule over the cork (the foil on top of the cork) to make it look especially professional and to add a proventive measure in case the cork flies off.