For this Project you are going to need:
4 skateboard bearings
a 3D printer
a wood file
the design attached below
Step 1: The Base
For this step you are going to have to measure your ball bearings then scale the model to the right size. After 3D printing, the bearings might not fit so you can take a file and widen the holes.
Step 2: The Caps
Now that you have put the bearings in we need to prepare the caps. Take a tiny wood file and shave the inside edges until the barely fit in the bearing. Don't make them too small or they will fall out on contact.
Step 3: You're Done
In this picture I don't have any caps but that makes it easier for tricks. I hope you are as proud of your spinner as I am of mine.