This is a blank Decora wallplate insert, to cover an empty hole after you remove a device like a switch or plug.
Design notes:
The depth of the front part of the insert should bring it into near-flush alignment with your faceplate. I've tested it with Lutron Claro and Legrand hidden-screw plates.
The top and bottom holes are threaded for a 6-32 machine screw.
Printing tips:
Print as-oriented in the .stl file, the larger part down.
Other notes:
If you'd like any adjustments (e.g., thickness of the front or back parts, etc.), just message me. I used OpenSCAD to design it, and it's fully parameterized for easy changes.