make a 3x3x3 from almost any shape even if it has a hole!
based on the void cube. Stl's are for example. I call the examples the "ghost void cube" and "bumpy bead".
for me I used a raft and a CCTREE build surface.
3 shells works well.
Use tolerancing in your slicer!
I used -0.2 horizontal expansion in cura 3.6.
Also, keep current orientation.
A little grease helps it turn.
I used PETG at 240c for the first version. Now updated to ABS+ at the same temperatures with a lower layer height. But other materials (like PLA) may work.
Assembles like the void cube.
Tip: a 1/4 inch flat head screw driver may help to push parts together. just be careful not to break the parts.
clean up & assembly
remove support material. and raft if used.
glue the pegs into the core.