Originally Posted by Deano56
this is what I am talking about here,
Looks vacuum formed to me. Most likely the plug has a shape that has a step along the outside to make it easier to cut. When it's vacuum formed the outside will be as tall as the inside, then it's cut to the current height, just like you will need to cut the openings for the head lights. It's been a while since I've had the equipment to do vacuum forming, but it's kind of fun to watch the plastic get sucked into shape.
Another option is to make a mold out of the current part and make a new one out of fiber glass. You would make the mold out of the outside of the part, and then lay fiberglass inside.