My Dad owns a cabinet shop, which is where I did some of the work when I built my camper.
They work pretty early over there, so nobody was in by the time I stopped in at the end of the day.
I was hoping a friend there could set me up designing a "hub cap jig"
Unfortunately, since nobody was there, I just had to help myself.
I decided just to keep it simple.
I traced out the shape of the ring on the aluminum, and cut it out with a jigsaw. (A VERY NICE and POWERFUL jigsaw!) I then screwed the aluminum to the ring, and then sanded the edge so the it wasn't rough, and also to actually get pretty darn round.
It's hard to tell in the photo (it was in the shade at the end of the day) but this is the shiny aluminum side. Actually looks pretty cartoony in the photo! Looks much cooler in real life!