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.
Radiused corners can be shaped by gently stroking the disk to create the curve you need. Steady the hands on the workpiece to make minute adjustments.