This cap clips onto the spool holder to prevent spools from somehow finding their way off of the holder and onto the print bed. I didn't think this was an issue but alas the ender has proved me wrong lol. The outer diameter of this thing is just big enough that no spool should be able to make it over.

Mk.1 Is featured in the pictures, It has a full cap on the off side. Mk.2 is easier to print as that side is now open.

If you print Mk.2 post a picture of it because I'm using Mk.1 and would love an example of Mk.2 printed