This is a basic FEMALE "dummy plug" for the XT60 connector. Unlike existing caps, this plugs into the male end and protrudes approximately two connectors' length with scalloped sides for easier handling. It was test-printed flat on the build plate of an Elegoo Mars resin printer and required no supports.
Cleaning Instructions for Resin
The two holes are small enough in diameter that solvent does not easily enter even in an ultrasonic cleaner. I recommend using a small paper dowel such as from a cotton swab with the cotton removed or other round object while the part is immersed in solvent.