These are XT60 Connector Caps that can be used to keep track of charged/used batteries, as well as provide a little more safety from shorts when transporting.
The design has tolerances for ABS that shrinks 1mm. Please size according to your material. I've included the .SCAD file if you'd prefer to modify from there.
Here's a YouTube video of the Design Process if you'd like to know more about it:
The author marked this model as their own original creation. Imported from Thingiverse.