This guy’s project is tremendous:

“Carbonating tap water to make seltzer is easy, fast, and absurdly inexpensive with my improvised apparatus. All that is required is to place CO2 gas in agitated contact with chilled water for a few seconds. In this essay, I’ll show you how it is done with easy-to-find parts and common PET soda bottles. I’ll also explain the kinetic chemistry of why it works so well. And in the second half of this essay, I’ll explain how I progressed from this improvised apparatus to installing a complete soda fountain in my home.”