Thermoforming is also called Vacuum-Forming.
Here is a DIY simple way to vacuum form a miniature car body at home found on YouTube.
The mold cam be made of 3D Print, CNC Wood Router or sculpting by hand using high heat plaster. Make sure surfaces are smooth and a minimum of 1 degree angle for part release.
If you want a durable body I recommend using Polycarbonate (also called LEXAN), more expensive and little harder to vacuum form than PVC or PETG.
https://www.youtube.com/watch?v=QuOAk0pgsGY
https://www.youtube.com/watch?v=Gx66mS7U2vY