The Fusor stuff should be good for what you are trying to do, if you are applying it to the right kind of plastic. I think that there are different models of Fusor are for different types of plastic, just like the "Duramix" product. Look on the back of your trim piece, there should be a code stamped into the back as to what kind of plastic it is made of. "PE" would be Polyethelene, or "PP" would be Polypropolene, there are several types of plastic. make sure that the filler you picked up is for the right type of plastic. Link discussing it:
View the 6th and 7th posts down.