Do NOT back tape and fill. I tried this when I was filling spoiler holes on a hatch. They just fell through when I started sanding. I wound up putting a single sheet of fiberglass on the back side, which was very easy. Then I filled the top side with Bondo Ultimate. After filling a dozen holes, including the wiper hole, you couldn't tell there had ever been anything there before. (except one of the spoiler holes had a reverse dent) I hit the biggest hole with my fist to make sure it was solid, too. In hind sight, I probably could have glassed the whole thing. As long as you build it up from the backside and spend some extra time shaping/sanding the top, it shouldn't be hard to get it perfect. It was my first experience with fiberglass, bondo, and body work. If you're not afraid of screwing up, give it a whirl. If not, I'm sure someone who has glassed before wouldn't have a problem with it.