Summary

I started from the "5-spur-fidget-spinner", removed the cut-out, created a cylinder to apply local specific density via Slic3r in order to make the one in the picture.

Which I find surprisingly better than the counterpart using 3 bearings as external mass (here it's only plastic). Maybe because the heavier mass acquire more momentum but also lose it more quickly, while playing with the infill density is just enough and more efficient in the end ?

Anyway, as the demand of spinners was problematic with the bearing supply at the time, it's probably a good idea to use 1 instead of 4 bearings (resource wise) :P