Here is how I make a cap for a soda bottle gravity bong:

Materials: soda bottle cap, metal pipe, thumbtack, scissors. If you don't have a metal pipe, buy one from a head shop for $5 (you can also order one online if that's not an option).
1) Take apart the metal pipe and screw the bowl piece onto the piece you put your mouth on.
2) Make a hole in the center of the soda bottle cap with the thumbtack.
3) Open the scissors and put the end of one blade on the hole.
4) Twist the scissors around and around, making the hole wider.
5) Stop every once in a while to see if you can't jam the bowl/mouthpiece in the hole. Eventually you will find just the right size where it will break through with enough downward force, creating a near-airtight seal.