Some thoughts on cutting fiberglass:
1) I set heavy-duty vinyl tape right at the edge of my cut, then scribe the edge of the line with a sharp utility knife. This helps keep the chipping of the gel coat to a minimum and stops the cutting tool's feet from scratching the gelcoat.
2) Beneath the gel coat are glass fibers that run every which way and that, causing your cut line to wander. Since you're going to cut the center of the hole out anyway, try screwing some furring-strip guides into the center at the right distance from your finished cut line, then use that as a guide.
3) My tool of choice: a Roto-Zip cutting tool. Much more power than a Dremel. My alternate is a "saber saw." both have a foot one can press against a guide, and the Roto-Zip can be purchased with a nifty little hole-cutter tool that's great for making proper radius curves for windows
4) I have never managed to cut a perfectly straight line in fiberglass.
5) There will be A LOT OF FIBERGLASS DUST. Ever got fiberglass in your skin? Fiberglass dust is even more dangerous when inhaled. Wear a NIOSH respirator to keep the dust from getting into your lungs where it can cause all sorts of problems.
6) There will be A LOT OF FIBERGLASS DUST. It hangs in the air for hours and gets everywhere. Cover everything with plastic drop sheets from the paint
section, then let the dust settle for several hours before pulling the plastic sheets off. Better yet, if the thing you're cutting can be easily removed and taken outside, do it.