If I were doing this repair I would span the holes with fiberglass resin and cloth. I would then apply fibered bondo and while that was still wet I would push a piece of steel mesh as I used on my panel in another thread, into the fibered bondo and then bondo over the mesh . This will give you added strength as the fibered bondo is twice as strong as their regular bondo according to the company, and the mesh will add much more strength.
Having said that I will post this video and keep in mind this is just regular bondo or equivelant and does not have thefibers or the steel mesh in it. My method would be much stronger than this -
I will also like to add that you can sand any areas that shows underneath ( you already have rust reformer there ) where the fiberglass may not be even with the surface . The fiberglass resin,cloth was only to keep the fibered bondo from coming through till it set. You can then give it a shot of primer and paint or a primer/paint product and then spray a bit of undercoating.