When i was in high school we did something like this for science olympiad.
This page could help you out
http://ourworld.compuserve.com/homep...ncherhbase.htm
When I was in highschool we made our base out of metal and had a "tong" shaped peice go over the lip of the bottle opening to help hold it in place. Just be sure not to make it too tight, or when you yank it to launch to rocket, it will just the entire rocket toward you and launch in your face.