i used a dremel and have a set of cutting discs ( bought at Home Depot ), one of them is for plastic. before cutting, i put masking tape on the fenders and drew the line. the tape also helps getting a cleaner edge. one thing you have to be careful with: when cutting the plastic, it melts a little. try not to cut the entire fender at once, let the cutting disc cool off once in a (short) while. after cutting, i carefully sanded with 1000 sandpaper. just be careful not to scratch the shiny plastic.
Nice phantom! How did you cut your rear fender? I was trying to do the same with mine a while back but I whimped out on cutting the plastic and just mounted my LEDs vertically. I'm thinking about getting a second fender to mess around with so I can try something else.