Help with 86 spoiler

I've got a 87 model and while at a salvage yard I found a 86 with a perfectly good spoiler. I removed the spoiler along with the third brake light and the rear wiper arm. I like the way the 86 wiper arm looks and would like to use the spoiler on the car. The problem is how to fill in the space for the third brake light. I thought about using the brake light for the spoiler but I really like the high mounted light on the 87 and want to stick with that.

It looks like the spoiler is made of abs plastic and I'm wondering if it would be possible to attach some type of support inside the opening and the use fiberglass to fill in the opening. Will fiberglass even adhere to the plastic or would it be better to cut some abs plastic and use a strong cement and use a filler to smooth out. I hate to sound dumb but I've never tried fabricating something like this and i'm hoping someone here may have done something like this before.