I was asked to put up a recipe for a CO2 generator that I used for a few years.
Using a 4 L milk jug that's filled about 2/3 full of warm water, add 2 to 3 cups of sugar and 3 tbls. of yeast. You can also add a tbls. of baking soda. Shake well with a lid on and then take the lid off. CO2 will start to form the next day. Put this in your cloches but make sure you shake it twice a day. They last for about a week. Empty them out and restart again. Remember CO2 is heavier than air so open your cloches from on top. One design that I saw used aquarium air hoses siliconed into the jug top and the hoses then positioned over plant so the CO2 falls over the plant.