Shave Mazda Taillights ?

Alright this is my frist time doing taillights alright and i got taillight fillers but they don't have the top body line. How should i go about building up the top body line. I got them fully welded up and grinded down now all i need to do is get the top body line straight. Oh yea is it alright to use Fiberglass trying to build the top body line.