An angle grinder is good for the job, just put a normal metal disk on it.
Just be carefull no to overdue it
Mark the square hole you are planning to make with a good clean marker. You need to be able to see this, even when you have dust all over the place.
When you have finished with the angel grinder, you can remove the last piece on fiberglass with a 3 mm power drill.
This will not work for the round holes
EDIT. Just edited a little misspell angel - angle damn, I guess I will start posting in Danish