Here's what you could've done differently.
Take a socket wrench bit and a 2 litre. Cut a small hole in the top, and wedge in the socket wrench bit. Make sure it's in there securely.
Cut off the bottom of the 2 litre. leave the whole label though.
Get a pitcher large enough to fit the 2 litre in, and put some ice but mostly water inside it.
OPTIONAL - put in a ping pong ball, trust me, if you have one just put it in.
Then, put in the 2 litre without the cap on it.
Fill the cap up with weed then salvia (or use a screen if you don't want to smoke weed with it) and then screw the cap on the 2 litre. As you pull the 2 litre up slowly, light the top of the salvia. The flame will be pulled down, the smoke will collect inside the 2 litre. Then you unscrew it when it's fully pulled through, and you slowly inhale the contents while lowering the 2 litre back into the water.
If you can inhale and hold in the smoke long enough, salvia should work fine for you.
You may ask, why put in the ping pong ball? Answer: to prevent water from going into your mouth, which might make this experience less than pleasurable the first time you hit a gravity bong.