Eureka Mignon Portafilter Hole Plug

 Ticks all the boxes:

• Stays in place with two snap connectors.
• Uses minimal amount of material.
• Is almost flush with the surface, but still is easy to remove.
• Prints witnout supports. 

The other plugs I found were only a pressure fit and their shape was too pronounced. This one is really subtle, easy to print and easy to manipulate with. My print is PETG 0.15mm layer, but should work in virtually any material and any settings.

Onshape project here.