If you only want to cast a few parts the use plaster. If you need to cast many parts you use fiberglass for the mold.
You first coat the original part with vasilene then plaster or fiberglass which even you need.
When using fiberglass for the mold you need glass mat.
when using plaster you can use burlap to help strengthen the mold.
Plaster is heavy but works great if only casting a few parts.