The casting cured faster than I expected, and turned out pretty good!
The two problems I see are:
The mold isn't done quite right, so the casting is about 1/16 inch longer than the original, caused by the impossibility of shoving the original part into the half mold all the way in - trapped air in the recess stops it from going in all the way, resulting in a longer casting than the original.
And, there are a couple of air bubbles, not in critical places, but demonstrating that the mold isn't quite right for allowing the bubbles to float out as the resin is poured in.
There are a lot of things to learn about how to do this well. I need to decide whether to make another mold, or live with this imperfect one.
EDIT: I decided to continue with the current mold. All it does is thicken the diffuser area, which I was intending to do eventually anyway. So, I just finished casting a new body, this time with white pigment in the resin. That worked out very well procedurally.