id say use something like this in the link.
its designed to be flexible and work with plastics. Not all epoxys work %100 with plastic and regular bondo would get hard and could crack or pop out.
i used a similar product on the rear bumper of my Monte SS and on my grand prix to fill significant holes/ cuts. adheres great and sands easily.
http://www.tooltopia.com/mar-hyde-33...googlebase_18u