It's not the best gravity bong, it's the best homemade bong...
First:
Cut a small outlet hole in the 2 liter for the downstem.
Second:
Cut a hole in the center of a cap with the diameter to fit your bowl piece.
Third:
Affix a straw as a mouth piece to the top of the liter, cutting another hole.
Fourth:
Construct the bong; fill with water (putting diffusers in the straw used as a downstem)
Fifth:
Smoke that shit good my brother.
Donezo; fb360 school of smoking