Just got these last week and already can tell you they are worth twice their weight in gold. One of my biggest frustrations with my glass carboy was getting the airlock bung to stay in the carboy as it always wanted to pop out on me. These fit over the top like a glove and they aren't going anywhere until you want them to. A++
I tried a couple of these last year, and just ordered a bunch more last week. These hoods slip over the mouth of the carboy and provide a more secure and more airtight fit than the regular carboy caps. And less hassle than trying to keep rubber bungs in place. Slide it on, add airlock and it's there to stay. These hoods are perfect for blowoff tubes, too.
I am very happy to be the first to review this great carboy hood. It fits securely and entirely over the tops of all my glass carboys. I have a couple of really old carboys from my winemaking father in-law that none of the other hoods fit onto without leaking all over the outside of the glass and onto the floor. This is the answer. I will be buying another one in the future.