Good point, you’ll need some strong servo for that task. Unless the leg-segments are locked in the sphere position so that the servos doesn’t need to take all the load in the roll mode. Good luck with winding a sphere!
Yeah, thats the picture I imagined. (Probably a stupid question to ask if you’ve seen that movie… 
)
I know that feeling very well…
Another thought, and I’ve not done it, although I have the proceeds, is to vacuum form a hemisphere. NASA makes them for the camera “bubbles” on observation planes. A plastics shop could probably do it for you. Or make up a frame to pull a vacuum on a sheet of warmed Plexiglas or Polycarb.
Alan KM6VV
Not sure if this would be what you’re looking for, but…
I think they also do spheres. I plan on getting a hemisphere for my R2D2-type bot which is in the works. Lots of sizes, not terribly expensive. Colored, clear, various thicknesses and materials, etc. It should be machineable, within limits.
So many ideas, so little time and money.
the videos are great. Its nice to see it coming together and moving around
