smoothing a plastic bumper> what to use

i want to smooth out the bumpers buy cutting off the humps. whats the best way to fill them back in? i know fiberglasss wont work on plastic so is there a glue or something you can buy for plastic? also what can i use for a filler plate? does it need to be the exact same plastic as the bumper? i guess i could find a wrecked hhr bumper and cut out of it. 
i have seen some places that advertise fixing plastic bumpers, should i just take them to a pro?

You can use the plastic epoxy from hobby shops or some body supply stores carry it as well.  Its a 2 part epoxy with an accelerant.  Sorry I don't know what its called.

go to a bodyshop supply store and get some Fusor, or 3m adhesive. It is designed for repairing all styles of thermoplastics. You can probably use any old bumper for your patch.....just go ask around a bodyshop for an old smashed bumper.....they should be able to help you out. -Brad

Fusor works really well.

thanks for the help. im gonna fill in all the side markers too. there are reflectors everywhere on a new car. is there a law for side markers if they are just reflectors? all i should need is headlights taillights and turn signals right?