Vented Cap For M16 Screws, ID 15.2mm x OD 22mm, 25mm Long, Silicone (Green), Pack of 10, HLBS16-P
SGD $25.70
Vented caps have a ribbed internal structure to make attachment easy onto axles. They also have an exhaust port that can be utilized with simple drilling. Externally, the step design make vented caps easy to remove.