help with speaker installl

Hello,
I am getting ready to upgrade the stereo and speakers on my starcraft 182. It came with two cheapo 5.25" speakers and a crappy head unit. I am replacing with an Eclipse head unit and some 6.5" Polk momo's, and will probably add a pair of Polk 6x9's as well.
My question is this- in order to enlarge the current speakers holes and make new ones for the 6x9's, I will obviously have to cut the boat. I will be cutting through fiberglass with gel coat covering. I have a jig saw with a special countertop blade (for cutting formica). Would that work? Otherwise, I suppose I could use my Dremel. I am looking for the best choice so that the gel coat doesn't chip. Any advice would be appreciated, specifically from those people with experience doing this on their boats.

Dremel with a Roto-Zip type blade works great. Just be careful it's easy to nick upholstery in the blink of an eye...

I would cut about 1/4 inch inside the line you need to cut then go back with a drum sanding bit. The sanding bit on the dremel cuts thru gelcoat fiberglass like a hot knife thru butter.

If you can have someone hold a shop vac up under yoru blade while cutting...ti greatly reduces the amoutn of cleanup and dust.