The design was kinda arbitrary. After deciding to use the 22 mm bearing, I found six 1/2" hex nuts in my junk drawer so I worked around those dimensions. The overall diameter (3.88") is on the large size for spinners. If you are building this for your kid, you might want to do some finger measurements first.
All joints are press/interference fit (no glue). The part printed slightly smaller than the CAD file so I had to spend about 5 minutes using a file to make everything fit.
I've included the STL file in this step. I've also included a STP file in case you would like to make modifications to the design.