The four gears mesh for a nice fidget.
I saw this concept on Vladimir Murano's Desktop Makes.
The assembly is held together with light press-fit pins into the mating holes. I used an offset of 0.35 mm for the diameter which provides a nice press fit.
Skateboard bearings (22 x 8 x 7 mm) are available from Amazon, 10 pieces for $6.45.
The author marked this model as their own original creation.