This is the 19,999th thread on this subject! But, maybe it will help someone. And, maybe I will get a Pulitzer Prize!
I'm making a two bottle, 2 liter bottles, DIY CO2 system, with a powerhead (internal filter) as the diffuser. Right now I am still waiting for the check valves I ordered on Ebay - 20 days since I paid for them, but that's another story.
I am using two club soda bottles that sat in our pantry for about 2 years, long enough to be pretty sure my wife won't miss them. And, for a bubble counter/yeast filter I'm using a 500 ml bottle that had sugar free, zero calorie lemonade in it.
The club soda bottles get a 3/16" hole in the cap, drilled from the inside out to try to avoid tearing the seal inside the cap.
A short piece of air tubing, cut at an angle goes in the hole. I poked the sharp end in the hole from the inside of the cap, then used a pair of needle nose pliers to pull the tube into place.
The second bottle was done the same way. Notice that I drilled the holes off center, since there is always a little raised spot on the center which can make the drilling harder to do.
My bubble counter was made the same way, except with two tubes in the cap, one long for the CO2 to enter, under the water in the bottle, and one very short, to let the "filtered" CO2 out, heading for the diffuser in the tank. I picked the lemonade bottle by searching the grocery story for bottles with larger than usual caps, to allow more room for the two tubes.
I have a tee fitting, from the LFS, in the inlet tube, so I can hook up two bottles of yeast/water/sugar. There will be a check valve at the outlet from each bottle, so I can disconnect a bottle for replenishing the mixture without losing much CO2 or introducing much air in doing so.