Tobra beat me to it. I'd do the aluminum as you suggest, (be sure to chamfer and round all edges), and I'd tape off the paint as shown and fill the voids with fiberglass cloth and resin, and sand smooth. That should keep any of those edges from cracking - you might even be able to epoxy the pieces of fiberglass that have separated back into the main part of the tail to make it a little easier to fill the voids.
