Nobody ever said you can't deform the rest of the bottle. I'll leave the research up to you. Let me just mention that bottles make a dangerously loud noise when they explode under high pressure. A simple parachute design is what kept us close in competition to the winners who achieved massive thrust.
Balance means everything.
Consult your physics, thermodydnamics and fluids books for rocket propulsion equations. In general the folks at craft and fabric stores love these kinds of things. They love seeing kids in their stores too, I think. There are many web pages that offer real help.