I managed to get my first successful rotocast of silicone rubber in a plaster mold. I applied 9% silicone thinner to the liquid silicone and two drops of accelerator before pouring it into the two-part plaster mod. It took about an hour if slowly rolling it around in all directions until the silicone was cured enough to leave it alone. I should probably use a little more accelerator next time. Silicone was mixed 90A:9B (measured in grams). It came out of the mold looking like a child's toy rubber ball, even full of air. Still, there is a blob of silicone on one side that didn't get evenly distributed. I suspect that it was the result of the mold sitting on the table while it fully cured out. More accelerator and a real rotocasting machine should take care of this.
This evening, I also was successful in milling out the bottom PCB cap for version 4 of the ball support. Unfortunately, the motor on my spindle failed, so I won't be able to mill the other half of this support until the new motor arrives. I also bought two more spares for next time. They are only about $18 on Amazon. The motor started to fail about half way through the job by running much slower and losing about half its power. I slowed the job down to about 30% to take it easy and try to finish out the job. I managed to get down to about two minutes left on the finishing pass (after 1.5 hours) and the spindle came to a full stand-still. I was able to manually finish the part with some sanding luckily, so all was not lost.
Picture below includes the rotocast silicone ball for the hand support, the version 4 ball support cap that covers the PCB and the latest PCB that hasn't been populated with parts yet (shown inside the newly milled cover).