This is a replacement cap for a spring-style doorstop. It is designed for a spring with a 10.8 mm outer diameter and 6.7 mm inner diameter, which is standard for my home. The centre post feature helps keep it in place and also makes it printable without supports.

Print it with a flexible material for maximum functionality.

This is designed in OpenSCAD and fully parameterized. I've included the source file if you want to tinker with it.

Update: Added a new version (r3) that has a hole in the middle. This might be less of a choking hazard for a small child, but as always, use with adult supervision only, take a CPR course, and do not allow your children to inhale these things under any circumstances.