A sphere trapped inside the edges of a hollow cube. The sphere won't fit through the cube. But 3D printers let us create shapes like this by printing them in place. The cube is pocket-sized at 25mm edge length.

You need a brim around the sphere to keep it from rolling during printing. The .3mf file includes this, but if you use the .stl file you'll have to add it yourself.

You shouldn't need to worry about anything else. I didn't use supports and the bridges (top edges) came out fine.

I originally modeled this in tinkercad, but I'm learning OpenSCAD and I now also provide a parametrized OpenSCAD model. You can use this to make the model any size you want (or see how it's made).