DIY CO2 is so simple I don't see a need for a model. I use a 3 liter cranberry juice bottle, with a handle, for a 45 gallon tank. It provides plenty of CO2 for that size tank. I used a drill to put a hole in the center of the cap, with the hole about 1/16 inch less in diameter than my tubing is. Then I cut the end of a piece of tubing at a sharp angle, using a utility knife. I poked the sharp end down into the cap, and used needle nose pliers to pull it down so it was about an inch below the cap. That makes a very tight seal on the tube, and didn't completely close the inside of the tube. The other end of the tube goes to a bubble counter, which is a small bottled water bottle. The cap on that bottle, I drilled with two holes, same as above. I pulled the tube from the 3 liter bottle down about half way into the water bottle. Then I added another tube in the other hole, pulled down about an inch. The other end of that tube goes to a check valve to allow flow out of the bottle, but not back in. On the other end of the check valve is another tube that goes to a small air stone, which sits in the tank under the inlet to the canister filter.
The mix I use, which gives me 3 weeks of CO2 before the bubble rate drops to about half, is 3 cups of sugar, 1 teaspoon of baking soda, and 1 teaspoon of bread machine yeast. I add the sugar and baking soda first, half fill the bottle with barely warm water and shake it to dissolve the sugar. Then I put the yeast in a cup of barely warm water and slosh it around to wet the yeast, then pour that into the bottle. Finally, I fill the bottle up to about 3 inches from the top with more barely warm water, add the cap as tight as I can by hand, and it is ready to go. The bubble counter gets just plain water, about 2/3 full.