A simple plastic piece you can insert in a door bolt hole (as long as it's rectangular with rounded corners) to prevent the door from closing. That's about it. Customize it to fit your door exactly.

Caveat and disclaimer: This was originally hacked together with a fidgety 1yr old on my lap who, at the time, was more interested in playing with doors than in OpenSCAD. Use your own judgement regarding safety! At very least: dimensions (must be tight fit into bolt hole) and print orientation (you want impact forces parallel with layers, not perpendicular) are very important. Although this has since been lightly tested by said 1yr old, don't blame me for any missing fingers etc. :)

Measure door hole dimensions with calipers, adjust for fit, and print at appropriate orientation.

If your bolt hole has a different shape, feel free to remix.