Following on from my Parametric Dodecahedron Fidget Solid, I made another one that's a regular cube.

This is totally parametric -- edge/corner radius, bearing radius and depth, and overall cube size are customizable, so if you want a giant cube with sharp edges (drop the radius to 0.0001), you can do that. There's also an option to have (or remove) a central bore hole that goes deeper than the bearing hole depth to keep the back end of the bearing where you hold it freely rotatable. All the config options are clearly annotated in the OpenSCAD source with a bonus ascii-art dimensions illustration. If you're rendering your own, check out Extending/Editing below for an important note.

Assembly

When you assemble, I strongly suggest finger-pressing in just the edges of the bearing into the hole to make sure it's even (if you have support cruft you might want to scrape out the bearing hole with a craft knife), then inverting it on a table so the cube sits on top of the bearing, then lean on it with your bodyweight to press the cube down onto the bearing to shove it in. Make sure you do this centered over a table leg -- some tables might not take kindly to bodyweight on them without a support directly underneath.

It may be worth printing just the first layers up to the end of the bottom bearing hole to make sure your printer is dimensionally accurate enough (or do a dimension check with a calibration cube). It's a darn tight fit but easy to adjust to account for printer miscalibration in the OpenSCAD config by expanding radii or depth.

Supports

You should print with supports for the bottom hole, at the least, as that's just a flat overhang. Most bed-only supports will shoot something up to the side holes; your bravery with the overhangs is in your hands.

There is a fudge factor for bearing depth set at 7.1mm compared to 7mm for normal bearings; this gives you some wiggle room removing supports from the bottom hole.

Extending/Editing

This includes the OpenSCAD source. Make sure you're on a recent nightly, and ensure that manifold rendering is enabled in your preferences -- this is liable to be a slow render if not (as CGAL backend is dog slow with the loops in there even though they're simple).

There's some trig used to generate the points for the cube and the associated offsets -- we can't use handy cube constructions since we hull() the vertices (spheres) to make it rounded in a predictable and reasonably mathematically simple way. I've tried hard to add good documentation and use sane variable names (LOVE the OpenSCAD community but man some of the source code feels like the developer was paying per-character-usage), but if anything is unclear drop a comment and I'll do my best to answer.