This is an idea I had when thinking about printing one part contained by another. It helped me learn a bit more about CAD in the process. The truncated sphere on the inside should spin freely within the cubic cage.

It prints quite easily but I'd recommend printing it at 100% (60mm each side) because if you scale it down the gap between the parts will also be scaled down and they might fuse together (ask me how I know).

The two parts won't necessarily appear one inside the other when you bring the STLs in to your slicer; if that happens, adjust the origin of one to be exactly the same as the other.

The OnShape model is here: https://cad.onshape.com/documents/0ce5679b54a54d67e927282a/w/b93025279a20b0d5bbda6f98/e/633744981db981c6dc3eb224

If you print one with the cube and sphere in different colours, please post pictures!