Carboy Cap
Details
The universal cap replaces several parts - it fits carboys of slightly differing neck-sizes, thus eliminating the need for several different sizes of rubber stopper. It also has mounts for either an airlock or a blow-off tube, allowing you to go back and forth between the two without exposing your brew to excess ambient air. The larger fits most 3 and 5 gallon carboys while the small fits most 6 and 6.5 gallon carboys.