Plastic grommet (aka nut) for my 1987 Toyota 4Runner. These plastic pieces are used to screw machine screws into sheet metal holes. For example, my 4Runner's side marker light is held to the body metal with three screws and three of these plastic gizmos.  

Base is 10mm square. Screw hole is 3.5mm diameter.  

I originally uploaded this to Thingiverse in August 2014, I'm publishing it here too.