That hole is too big to fill using a backer. And once you blast and cut back to thick, stable metal, the hole will grow. Easiest way is to get a donor piece from someone replacing a full quarter. Or use a brake and a shrinker/stretcher and make your own patch. A form will be needed to make the trunk bumper contact portion. Judging by that rot, I'll bet there are other areas on the car that are just as bad.