I saw a really cool set up on the Discovery channel once where this guy had a length of hose connected to the neck of a 2-liter coke bottle.

He had a small hole poked in the base of the coke bottle so that when he sucked on the bottle the fluid started to collect in the bottle instead of his mouth (as I've had gas do to me once ---blech!!).

Never tried it myself and using the manufactured ones are definately safer but if you're in a crunch ....