Bondo body filler is polyester putty. It's cheap and works great.
> Spitfire7
What you can do is put a tape on the front surface, and fill the areas with polyester putty from the inside. Once the putty is cured, remove the tape, and the putty should be flush with the bumper, saving you time to file and sand the putty. After finalize the shapes of the two openings using a needle file, apply Tamiya's basic putty very thinly or use Gunze Mr. Surfacer 1000 to blend the puttied surface and bare plastic. Then, smooth the areas with the sand paper.
When using different materials, make sure to prime and polish the modified areas, or the difference in texture will be noticeable after the body is painted.
Good luck.