There is a KC / Topeka forum where they several people have made their own....they had this huge Frag Swap meet and greet where one of the events was how to make them...try looking them up....
I saw one, and all it seemed to be was a 2 liter coke bottle, an air pump, and.....can't recall the rest...but very very very simple to make...I just didn't really pay attention when I saw it. Someone even made it out of those plasic CD storage cases (where you buy lie 100 blank CD's).....