Rubber Fermenter, Carboy, Demijohn caps
Rubber caps to fit fermenters, water bottles, carboys and all many of things. Mainly for glass carboys but fit plastic ones as well.
You'll want the 50 mm one for those water bottles out of the cold water machines.
Come in three sizes, they stretch quite well. The 50 mm will fit up to a 70 mm neck glass carboy at a stretch. So there's a decent overlap on the sizes. The 60 mm rubber cap fits the 70 mm carboy much better!