I was afraid that you were going to ask that question.
To begin with,I am not a perfectionist but close. The crests have a rounded configuration that is not the same as the baby moons. The first thig I did was to make a full scale mock-up (wood)of a baby moon on my lathe. Then I flattened an area on the top having the same diameter as the ctest. I placed a baby moon over the mock-up and using a stout piece of wood (a piece of a four by four), I hammered the flat onto the baby moon. Since the mock-up was the exact shape of the underside of the baby moon, the flat was exactly centered on the baby moon. This was the easy part.
The crests come with three attaching pins on the back side. I made a cardboard impression of the location of the pins within a circle that was the exact diameter of the crest. I transferred the hole pattern to the baby moon centering the diameter of the cardboard impression with the flat on the baby moon. I then used a greese pencil to mark the pin hole locations on the flat on the baby moon. Holes are drilled at the marked locations. Some hand work had to be done to make the holes elongated which is necessary because the pins point inward on the crests. After all of this, it was an easy matter to attach the crests.
One last point. The baby moons were of course the cheap reproductions which are known to rust from the inside. In hopes of preventing rust, I painted the inside of the hubcaps with a two part epoxy paint making sure to apply paont inside the rounded over edge.
It was a lot of work, but the result was worth it.