Fidget Spinner Concentric Rings Cap/Cover
Update 6JAN2018: Although my original spinner caps do not come out on their own, they will come out if heated by the sun. I have posted a new design where each cap interlocks with the other. These never come apart (I have tested them for months). Once together, they must be broken to get them apart.
This is a spinner cap/cover that I include with all of my spinner designs. It has a satisfying "click" when you insert in to the bearing and isn't going to come out on it's own. It's fairly low profile and has excellent finger friction so it won't slip out of your hand very easily.
These are for 608 bearings.