You can't use a hood itself as the mold because then all the units you pop from it won't be the right size.
First step when creating a mold is to make a plug from which to get your mold. Since you can use an old hood as the plug, that part's done. You do have to prep the plug though. The plug has to be flawless in it's preparation since every single piece pulled from the mold is going to mimic it. It will need to be wet sander, primed, etc to fill any scratches, small dents, etc. Even tiny scratches will show up in the mold. After the plug is prepped with wax and so on, you can take a mold from it. The mold is basically prepared in the same way as a finished part would be except it is built up much higher and is braced using wood or metal for strength purposes. Now you have your mold form which you can pull finished pieces from.