RE: what is best for making molds .
If you have access to a lathe, you could turn it out of wood. Also, make it longer than the final cowl and cut it to the proper lenght. This way, if the plastic wrinkles as you pull your part, it will wrinkle on the part that gets cut away. Be sure that the part gets wider at the bottom so it is easier to pull the part off the form.
Scott