I cut a square hole in the base to insert the stick and attached rollers in front and back so the platform sits and rolls nicely:
This is how it fits inside the ball:
Then, I put the stick into the hole (higher than it should) and put the top of the body on pushing the stick with magnets to the level where it would touch the top of the ball. Then I would open it, push the stick ~4mm down and draw the line at the level where it should be attached to the base.
And eventually attach the stick with screws to the platform (yeah, one screw is missing):
In the end – looks quite fine and balanced:
You can see that the space for head-rotation engine is facing forward – I will counter the weight of that with the battery pack fitted on the other side of the stick close to the bottom of the base.
The extra stick under the base will be used as an attachment for a weight (probably a 0,75kg disc from a dumbbell).