The ‘sturdy’ in the plug comes from several micro-features:
- the plug is rooted in the main object’s infill,
- it has practically no infill itself, just many walls, caused by the ribbing and the cross-shaped cutout,
- and the bottom chamfer adds more strength at the biggest stress point.
The chamfers at the tip and the hole’s rim make insertion easy, and the outer hull of the plug is slightly slanted to increase friction the deeper the plug goes in, ultimately leading to a comfortable insertion but with a snug fit.
Recommended print settings: Arachne walls, and Gyroid infill.
The design principles were mostly lifted from this YouTube video.
See things/parts/joining at main · jhermann/things · GitHub for a STL and the SCAD file.