I've continued to experiment with tops sustained by rocking the base. At the moment, the 64g with 5/8" steel ball is about as good as any, though I've matched it with a top which has a similar Delrin body and a 3/4" tungsten carbide ball. The carbide density is about double that of steel. This ball, which costs $27, weighs 54g, and the entire top weighs 101g.
Both of the above tops can be sustained indefinitely by rocking them about 25% of the time and resting the other 75%. I like to rock for 20 to 30 seconds, then rest of 60 to 90 seconds and repeat. I'm running in the range between about 500 and 650 RPM.
The topple speed for the 5/8" steel ball top is just under 310 RPM. For the 3/4" carbide ball top, it topples very suddenly just under 270 RPM. I think the sudden topple is a clue that it's very well ballanced -- which would be expected because the carbide ball is near perfect and comprises more than half of the mass.
Best,
Alan