Done this years ago to get the shape of a half of a 100 watt light bulb. Used thin plasticard
a very hot top element on the kitchen oven .I also made a flat piece of plywood
as a base and 4 pieces of wood to secure and try and stop any air from entering into the area below the plasticard
that was going to be shaped. There was a hole in this area where I attached the hoover to suck out all the air, when the plasticard
was very soft from being in the oven. Done one or two experiments and found it best to use the hoover when the plasticard
was melted so it was sinking on to the mould of the shape on the ply. I am sure they still use ordinary plasticard
these days but back when I done this it was a little more brittle.