if i am really desperate . i have a bong now, but i used a 2 liter bottle,aluminum can, some tinfoil.
Step one: cut the bottom off the two liter
step two: take a knife and punch a small hole in the cap of the 2 liter
step three: take scissors and cut a rectangle out of the aluminium can, roll up the aluminum into a cone
step five: jam that aluminum cone into the hole in the cap. the tightness will hold the cone and keep it from unwinding. and it will be air tight.
step six : take a 1"x1" square of tinfoil and wrap around a pencil eraser, then push it down into the cone you made. flip the pencil over and poke a few tiny hole in the tinfoil to make a screen.
YOUR DONE YOU NOW HAVE A GOOD GRAVITY