I started by modeling it up in CAD:
I then cut patterns for the part and for the core box:
Start of the patern on the CNC
The two halves of the pattern
The core box
Making the mold
Making the core.
I had a lot of trouble getting this core to harden and come out of the core box. I was using Sodium Silicate and CO2 to harden it. The core kept breaking where the large mass on the end meets the small diameter. After about 6 tries, I ended up using two separate pieces. I had a core for the large end and a short core for the small end with a gap in the middle that got cast solid. I drilled it out later.
It didn't turn out too bad. There were some minor sink marks in the area where there was no core, but I filled that in with Bondo before it got painted.
A test fit on the siren. It fits good!