Here is a set of 5 Flush Mount Silver Plastic Body and Sheet Metal Hole buttons, commonly used to plug drain or access holes in sheet metal or car bodies.  There can be several other home, business, or industrial applications as well.

These are for a hole size of 1/2 inch, and the overall outside head diameter is 7/8 inch, 1/8 in above surface when inserted into hole.
See photos for details.