A laminate trimmer—basically a small router—fits into tight corners and tends to be easier to use when cutting vertical surfaces. The carbide-tipped router bit has a bearing at the top which rides along the template edge, cutting the fiberglass to exactly the same shape as the template—any slips will cut into the opening, not out into the surrounding fiberglass. Just be sure to keep the router base flat against the template.
Make the cutout in a couple of passes, the first with the router-bit depth set to cut only through the outer layer of fiberglass and part of the soft core. Increase the bit depth and make a second cut all the way through. This not only makes the router easier to control, but also ensures a smoother fiberglass edge.
Cutting fiberglass with a router can be tricky. If that’s a concern, cut around the inside of the opening about a quarter of an inch inside the line with a jigsaw, and then use the router to trim off that last quarter inch, which makes the router much easier to handle. It adds a step but results in the same smooth edge and precise cutout.