Flange Vented Cap For M20 Screws, ID 19.2mm x Flange Diameter 36mm, 25mm Long, Silicone (Black), Pack of 1, HLCS20-2-P
SGD $4.40
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.