This is what active fermentation looks like:
















When I first put this cover on, it was sagging, which is to be expected. After a few days of sitting, the yeast re-hydrated and started working their magic… converting sugar into alcohol and carbon dioxide.

My equipment kit came with a heavy, food grade plastic cover and an elastic band to cover the primary fermenter. A hard lid is also available for these pails. The only thing you have to do with a hard lid is cut out a hole where you can place a bung and airlock to allow the carbon dioxide to escape. If you don’t there’s a chance the gas might build up inside and pop the lid off. This system is nice because it’s quite cheap and simple. I do think I might buy a hard lid for my next batch though. They’re only a few dollars, and will offer a little more security if a curious feline happens to get too close.