Wizz wheel, which is basically a carbide cutting wheel. You can buy them for any regular drill, but if you have an air powered grinder it'll be lighter to hold and easier to control. You could probably even use a hack saw but you'd have a reall hard time getting it started, since you'd have to drill a hole or something. You can sand it down after it's cut and even get some bondo if it turns out rough and make it look smoother.
I'd suggest marking off the hole that you want to cut, and experiment with the cutting "inside" the hole, not near the lines that actually need to be cut, that way when you make your final cut you'll be cutting out any mistakes you make. This will give you a feel for what the blade is going to do when you make your final cuts. If anything it might splinter of you go too fast. My guess is you'll need to do some clean up stuff on the edges of your cuts reguardless of how you cut it, just to make it look professional and clean.
Fiberglass cuts really easy and goes smooth. Use a die grinder with a normal disc in it and cut it out. Tape the edges around where you are going, and start.
Made sure to wear long sleeves, and a mask. Working with fiberglass, espically cutting it is a very itchy job and you dont want it on your arms or in your lungs.