Ah, another "theoretical expert" - you're of course free to try to improve upon their design, but I think what they ultimately built is the end result of a lot of effort (and probably trial and error) already. If you look closely at the video, you will notice that the pieces are slightly V-shaped to recenter the ball if if deviates from a straight line, and as long as the ball is in the center, the tempo is correct. When it goes off to the side, it will be pushed back to the middle, but will inevitably be slowed down slightly.
I think there are many possible shapes and the difference is basically tuning. There are makers of wooden musical instruments a'la xylophone and they just cut it in fantastic shapes, different each time, and then tune individual parts by removing some of the material.
