Very nice. Would've like to have seen some pics of the process. I'm having trouble visualizing how you did that.
Check out this video.
. Its where I got the idea... Cant take credit for the design!
In short, I started with 4 pieces of 3/4x3/4x8" oak. Tape them together
VERY tightly with 2" masking tape, then turn a section of it round (the "inside" opening). Sand and finish the inside then mark the corners of the 4 slats and rotate 180 degrees so the void is in the center. Insert the balls, glue and clamp. After 24 hours very carefully turn to shape. I didn't know how much to turn it down so I just stopped often to check the thickness of the outside of the rattle part. When I was sanding the outside to finish I ran a piece of sand paper through the slats to sand each of them and make the sight window larger.
It was a fun project... but a little tense at times worrying it was going to explode on me! Needless to say I took extremely shallow cuts!