One of the photos shows the design evolution I started with Tinkercad and made a simple circle and made some grooves for a lock-washer to fit. I worked but needed more weight.
I then modified it to use 5/16" (12.5mm) nuts and changed it a little to use caps on the nuts it worked better but wasn't perfect. I switched to Fusion 360 and ended up with the final design which by happy accident looks like the 'Nuclear warning symbol' hence the "Nuke Fidget Spinner"
Hole in the center is for a 608 (8x22x7mm) bearing.
June 2 2017
Uploaded a new version of the 608 Bearing caps I noticed that the originals sometimes would break when trying to insert them fully so I changed the design the new ones work much better.
June 4 2017
Changed the Bearing caps slightly again for added strength and changed loading orientation to correct position so they don't have to be rotated also added a full plate version with all the caps and body
Also for those who may not know this, remove the bearing shields and soak the bearings in a degreaser (I use TSP, available in paint section) for 15-20 minutes. Dry and blow out bearings I went from 12-15 second spins to close to a minute spins after performing the cleaning (do not use any lubrication, not needed)
June 4 2017
I also removed the bearing retaining clip (part that holds and centers the balls in the bearing) mine were a plastic of some kind and easy to remove, removing it increased spinning time to 2 minutes!
Printer:BigBox Pro .04 Nozzle
Tolerances are tight for the 608 bearing caps so You may need to shave a little off the shaft to get a good fit on my printer they are tight but go in fine, your mileage may vary.