Fully printable balance board.

It has a diameter of 420mm and a height of 90mm.

Only 8 M8 bolts and nuts are required for assembly. However, I would recommend additional gluing. 

The connecting groove is tightly designed for stability. To ensure accuracy, I have provided supports here. However, these do not play a significant role in terms of printing time and filament consumption
 

What you see here is the test model of my printable balance board. I printed it with cheap PLA, hence the warping and slight inaccuracies when printing. Nevertheless, the stability surprised me, so I'm publishing it here in advance. 
My 18 year old son is already testing it.
Pictures of the final version in ABS will follow soon.

For printing, I strongly recommend PLA+, PETG or ABS.

To improve the stability, you can increase the wall layers and the infill.
The hardness of the hemisphere can also be adapted to your needs using infill.

As the strength and durability depends on many factors, use this model at your own risk.

Have fun with it

PS. If you have any suggestions for improvement, please let me know.