A bottle cap designed to fit 5L (~55mm) PET water bottles, with a hole (11mm) to accept an airlock. I have seen other similar designs (particularly DennyNC's) but they are for a smaller mouth bottle and doesn't work after up-scaling to the proper size. I just took danceyoga's design and added an 11mm hole in the middle. After printing the hole came out slightly undersized for me but it worked for my airlock. I'm currently testing it with a batch and plan to improve upon the design.
Things to improve: * The hole needs to be enlarged to better fit some thicker stemmed airlocks * The material around the hole is thin and flexes when the airlock is inserted, some strengthening may be required(although this might actually be a pro in some cases since this way it has a better seal without a rubber grommet and the material is strong enough to not break) * I plan on added a different version witch can accommodate for a rubber grommet providing a better seal (not strictly required but a nice option)