Start with a block of rigid insulation foam. The pieces are all glues together with "Great Stuff," which is a canned expandable foam meant for filling gaps. It glues foam very well, and doesn't dissolve it like a lot of glues.
Make it round. I used a had saw.
Carve into the side of it and mark the shape of the outer edge of the shell.
More carving with a saw, and with some very sharp 40 grit sandpaper. I was foolish and didn't wear gloves so it chewed up my hand a bit too.
Shape taking form. I had to add more to the top.
Oops I missed some pictures. But it was all carved and shaped with the saw and a couple knifes and some sandpaper. I cut the groves in with 80 grit sandpaper wrapped around a sharpie.
Here I've coated the whole thing with gray house paint, and then added some Bondo to the roughest parts to try to smooth it out a bit. Then I sanded the Bondo.
I did 5 layers of rubber, then plaster mixed with fibers. This makes a mold jacket.
Oops again, lots of steps skipped. So I made the other side of the mold, took off the plaster mold jacket, took off the mold (cutting it around the middle) and then cast the two sides in fiberglass. Then I used fiberglass to put the two sides together. Then paint it white, mask it off, paint the tan and red, and use an airbrush to add some detail shading.
And here's a video of it in action!