These are made from natural color food grade plastic. They fit a 38mm jug mouth which is about an inch and a half outside to outside, and about an inch inside to inside. This means that they fit standard 1 gallon and 1/2 gallon jugs. They work with all standard 3/8″ airlocks including the 3 piece, S type, Mini, and Double Bubble.
For the longest time we made our own caps by drilling a 1/2″ hole in a plastic cap and putting in a grommet. It was time consuming and the drilling was a little dangerous. Now we just reach into the bag for one of these and it’s done!