Decora electrical outlets and switches are the modern standard in the US.  If you need a blank insert, just buy a Leviton branded one - they are designed well and safe to use.  Use this model if you know what are doing and need to customize your own.

I needed to make my own insert, and was dismayed that nobody seemed to share an original design file that I could leverage.  So I modeled this myself in Fusion 360.  Also included the STL and Step files.  Hope this helps someone like me.

This model is accurate.  The only challenge are the screw holes for the plate hardware.  You can use a 1/8" drill bit to enlarge those holes, and slightly chamfer/debur the hole and those tiny plate hardware screws will bite and thread themselves.

BUT – I recommend pilfering metal threads from a commercial insert.  Use a soldering iron to remove them and insert into appropriately sized holes in your model.  I have Monoprice branded decora inserts that worked perfectly for this.  

The file unit is in metric, but many of the measurements are in imperial.

NOTE: Please know that PLA is not recommended for electrical boxes that house mains current.  PETG is a much better choice, but the safest are UL certified self-extinguishing filaments such as Prusament PETG V0.  See Video below.  I did use PLA myself (Bambu Labs White Ivory Matte) for the closest color match, but my application is 100% low voltage.