Someone made a post a while ago that was about this, making a grav bong out of a 2L bottle and a glass bowl and downstem that filters the smoke through water, like a real bong, but is used the same way grav bongs are used. The thread topic was something like "best gravity bong ever! cheeep!". If you search for the word cheeep (3 e's) you should find it, he had a pic of it and a video on YouTube of it in action. It's pretty sick.