By far the best tool I have found is a "Multi" tool like the Dremel Multi-Max. The three inch semicircular cutting blade (like the one shown) makes the thinnest and cleanest of cuts. Extremely easy to control and a minimum of dust!
BTW, when you cut your opening, take care to reserve the
fiberglass cut-out. You may find that this will come in handy as a patch, or even as an access panel hatch cover. Since it's original, you know it's gonna match up with your other
fiberglass.