Shaping fiberglass parts might be messy, but it doesn’t need to be hard. When I need to cut back an edge, I find that an angle die grinder with a 2-inch sanding disk makes quick work of the task. The key is to anchor your hand to make it steady, and then take shallow cuts up to the pre-drawn line. To make a radiused corner, the edge of the disk makes a great carving tool—just take it slow and steady for a nice shape. Always wear a dust mask when grinding fiberglass.
Mark the lines for the cutout you want, then use a 2-inch sanding disk to carve the glass up to the line.