A parametric hole cover/plug/cap/whatever! With an optional counterpiece!

Customizable parameters are:

• cap diameter, thickness and fillet
• plug diameter, length and wall thickness, hollow or filled plug
• counterpart or no counterpart and its diameter, thickness, fillet and plug clearance

STL files are not included. You need to use OpenSCAD or the Thingiverse Customizer to customize it and generate an STL.