I have a cautionary tale about any type of gap filler and it doesn’t matter if it’s plastic, rubber, metal or fiberglass.
See where this attaches to the spoiler with 2 screws?
Notice there is nothing holding the top permanently. Except on the outside end.
That is a small metal angle screwed in at the top. It also has a piece of double sided tape on it to hold the gap filler from flapping around.
You can see here that my tape is not holding the gap filler as it’s supposed to. And it’s a good thing it popped loose. The reason it’s not sticking anymore is because I hit the lower edge of the spoiler pulling into my driveway a little to fast. Not hard but enough to scrape and push the spoiler up some. When the spoiler hit it pushed up on the gap filler popping the tape loose.
If it didn’t pop loose and had been permanently attached along the upper edge with screws something would have probably broken.
So which ever type of gap filler you choose to buy do not attach it permanently at the top and bottom with screws.
So be warned.