Jim, Permatex makes a plastic specific glue that I used for seat back repairs, as well as some of the fibreplastic panels in the rear of the vehicle. I even used it to fix chips, and made up an edge that was missing. It's called Permapoxy 5 Minute Plastic Weld. I tried the GT Motorsports Plastex repair kit with poor results. However, they do have a block that can be heated and used a a mold. I heated it up, took an imprint from the other seatback, mixed the Permatex and filled up the damaged portion. I picked up the grain of the panel from the other back and it looks good! I twisted and flexed the back, and it's a rock solid repair. It dries in the same color as the bare fibreplastic material. Just food for thought.
-Keith