Dremel + cutting wheel will melt the plastic and would be difficult to cut out curved shapes. In fact, most rotary tools move so fast that they melt as much as they cut.
Jewelers saw is not easy enough to pivot and take a section out like this. You would have the same issue with shears, or other larger cutting tools.
Looking at the shape of the void, I would bet it was converted after assembly.
You would need a power tool with a thin blade to do this. I would think bandsaw, body saw, or possible a jigsaw if you could secure the piece well enough. After the initial cutting, you would go in with clipper and a grinder to vary the cut edges. That is the way I would do it.