How to get fiberglass to stick to plastic?

Well I'm hoping that I have my car painted by Monday/Tuesday since it's supposed to rain most of this weekend here, and that will give me 3-4 days to do some interior work before I have to go on a long road trip.
Now, I made a thread earlier about how I'm wanting to get my extra set of 6x9 speakers in the back of the car for a little extra power to the expensive CD player I got to replace the one that was stolen. I was thinking about just screwing it into the plastic, which would be quick and very easy, but I didn't really like the way it looked.
So I was thinking about making a small hill for the 6x9 speaker to be attached to on the panels, which will require a little fiberglass work(and wood cutting). My only problem is, how would I get the fiberglass to stick to the plastic?
Now, I made a thread earlier about how I'm wanting to get my extra set of 6x9 speakers in the back of the car for a little extra power to the expensive CD player I got to replace the one that was stolen. I was thinking about just screwing it into the plastic, which would be quick and very easy, but I didn't really like the way it looked.
So I was thinking about making a small hill for the 6x9 speaker to be attached to on the panels, which will require a little fiberglass work(and wood cutting). My only problem is, how would I get the fiberglass to stick to the plastic?