I used a pneumatic cut-off wheel to do mine. Basically its a die grinder with a 3" cut-off wheel and a shield. It cut through fiberglass with no problems and left a smooth edge. You do have to be careful not to let the wheel catch and go where you don't want it to.
Just a little trick for you. Get some 3/4 inch masking tape and use it to tape out your desired wheel opening line. Use the cutoff wheel as Mark said to rough it out to @ 1/8 inch of your tape line. Finish the last bit off with 36 grit on a hand sanding block (maximum control if your not used to this) or use a small grinder or air file with 36 on it if you have and are comfortable with them.