how to cut fiberglass?

i need to cut bigger speaker holes in some fiberglass on a friends crownline. i know there is a trick to doing this without messing up the glass. any advice would be much appreciated. thanks in advance!

I've used a rotozip or dremel tool with great success. I covered the entire area with masking tape and marked the template over the tape. Worked for me but maybe someone else has a better way.

if you use a rotozip/dremel make sure to accompany it with a strong vacuum, dust will go everywhere!

AMEN Tim! I had a helper with a very strong shop vac holding at the location while doing it, it cuts the dust down by 80% at least!

A dremel will melt thru fiberglass/gelcoat like a hot knife thru butter with a sanding bit on it. I don't mask anything. I mark on it with pencil and start "sanding".
A shop vac is a must.

When/if drilling, drill in reverse.

Rotozip will burn up blades so just know that, Router with a template and carbide pattern bit works the best but makes lots of dust. Just use double sided tape to hold mdf template in place and trace with router perfect cut every time. Pneumatic body saw works well. Body saw is my weapon of choice due to less clean up. If your fiberglass if fairly thick it will eventually melt the teeth on the blade. Finally you can use a jig saw with a fine cut metal blade just a little hard to maneuver in tight spaces. Used to do installs back in the day for a living.