Keith, a physics teacher in England told me that thecommonly available 22mm pipe at DIY chains like B & Q is too tight a fit on the bottle. He sanded down the outside until the 2-litre bottle neck would slide smoothly over. He said there was some slightly smaller diameter pipe, but he couldn't find fittings for it.

Here is a really interesting design from Europe using metric measurments. http://homepage.ntlworld.com/telescope/Rocketweb/Launcher.htm Jay Morgan, Cubmaster of Pack 327 alerted me to this free-thinking site. Instead of making a bump in the PVC pipe to make the pressure seal, he wraps PTFE tape (Teflon thread-seal tape)--not so it forms a big bump to jam the bottle against, but rather enough so the bottle neck fits fully over the tape. So you don't have to be quite so precise about where the bottle is on the pipe. However, I am getting feedback that after a time or two the Teflon tape rips.

By the way, correspondent Daniel Berry says that a kind of Mexican soda bottle works better than the kind sold in the U.S. I know some serious rocketeers intentionally heat and re-form bottles to be longer, and here it's already done for you.