Flange Vented Cap For M18 Screws, ID 17.2mm x Flange Diameter 30mm, 50mm Long, Silicone (Red), Pack of 1, HLCS18-P
SGD $5.50
Vented caps have a ribbed internal structure to make attachment easy onto axles. Also has an exhaust port that can be utilized with simple drilling. Externally, the step design makes it easy to remove.