CO2 rocket/ bottle boat?

[StephenW]

i am doing an experiment in my chem. class. where we have to take a bottle mix vinegar and baking soda to make CO2 and the race it in a gutter. i need a way i can drop in the baking soda and cap my bottle fast, i tried putting in the baking soda first than vinegar but i lose to much CO2 while capping the bottle, what are some good ways to get the 2 ingredients in fast and i can cap it fast?