Ok if you use the fiberglass, use a smidge less hardener. Grab a piece of wax paper and apply the fiberglass to the shiny side, that is the waxiest part. Now this is the craftsmanship part. A nice thin section on that wide mass of wax paper. You gotta eye ball that line . What you have so far, is a nice smooth layer on the wax paper side. That layer might want to be thin, not a glob. and apply. Thing is though, is that the fiberglass that is being on the panel side, needs something to keep it there and not start having it flake off so to speak. That surface might want to be a rough surface or might want to have also a hlle for the fiberglass to get into and mushroom so as to stay in place. You can do the same thing with some bondo.
The wax paper will peel right off. It is an art form, that if you try to move the placement, you will mess up the smoothness, and you will need to start sanding the cuts that form from the wax paper crinkling.
Me, on plastic, I would go with some sort of epoxy. less problems and less time and mess.
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