Is the dish to be made of steel sheet or mesh? Or are you going to use fibreglass or something? Is this a one off project or for mulitple dishes? The purpose of the mold is for a hot pour cast or as a reference for the shaping of a parabola?
As I don't know much anything about the purpose (class) or materials your dishes are to be made of, I can answer one simple point...you first need to make a master part (out of wood, fibreglass or something malaeble like aluminium). You then follow the typical cast/mold process to make your preferred end product of concrete medium.
Yeah, was a thought I had too...got one in my backyard covered under bush, waiting for a rainy day project. I think the postage may be a $$$ if the Op wanted it