Okay, I can't believe I'm adding to the oversupply of fidget spinners; but I've made these for friends and they are very popular and I haven't seen any like them. So here they are.

Personally I like the straight version but everyone else prefers the look of the curved version. Also shown is a (blurry) photo of one with multiple balls per arm -- go nuts if you want.

These are designed to use 3/8 inch steel balls which are readily available on-line. They are apparently often sold as sling shot ammo. If there is a metric equivalent near 9.5mm that anybody wants, let me know and I can add a version for that.

These use the usual 608 bearings.

I recommend printing this in PETG or ABS (or polycarbonate). PLA works but breaks too easily when they are inevitably dropped.

The original design is available from Onshape at:
https://cad.onshape.com/documents/875db54227b46569bce5cf3b/w/a9bc86a3641cb628e7b220ea/e/7eb588195532bac4b3c91d07
There are a couple other designs included there which work but aren't as entertaining.

Use whichever spinner caps you prefer. Shown are my favorites:
www.thingiverse.com/thing:2204246 by scotty13

EDIT:
and my new favorite:
www.thingiverse.com/thing:2314895 by vlevit

EDIT 2:
Added a version for 8mm balls by request. Untested.

EDIT 3:
Added a version for 1/2 inch balls by request. VERY UNTESTED. I don't recommend you make this unless you already have the 1/2" balls and want to give it a shot.
UPDATE: Based on a make by McUsher, the hub on this one was too thin. Added "Revised' version with a thicker hub. I'm not deleting the old version until the new version is confirmed as working.
UPUPDATE: Based on cbsarka's make, the revised 1/2" version works. Old version deleted.