Strengthening the edges?

And providing a 100% guarantee the sphere can’t flip?

Well, adding a set of curved aluminium edges to the sphere is one cheap and easy way to do this. Extruded aluminium is only £4 a meter and the 20mm sections can be slightly curved and bolted to the existing holes on the panels. Some switches on the base can also provide a mechanical cut out if for some reason the sphere is over rotated, being triggered by these extrusions.
The sphere is also very strong but we might encounter cases where all the weight is on one side, stressing the sphere and possibly deforming it. By adding these sections we gain mechanical strength at a low cost.
We can also use similar sections inside the sphere to add strong connectors for heavy internal structures such as the chairs or foot pedals.
brace

There are also different types, so we can even use one that only has one grooved side.

Screenshot 2014-10-08 12.49.37