actually, You can make fiberglass, pretty much as strong as steel- it just requires you to use enough of it, and use enough resin. If I were you, I'd clean out the rust, POR 15 the edges which were exposed.
Then, you need to use something for support ( you need support even for the POR15 method. They tell you to use "duct tape" to hold the holes from the underside so that the POR doesnt drip out). I modified that method slightly for holes that are large.
Use thin tin/aluminum siding, cut to approximately 1" past the repairing edge. Rivet in place from below (all depends on how you feel about fiberglass- its strong stuff, its just how much faith you have in it). Use epoxy resin, brush on coat, with particular attention to the edge (make a seal, so that moisture cant come up). Use fiberglass MESH (POR 15's stuff is good, but its thin. You want the woven mesh, not the chopped up fiberglass. Woven is stronger).Make this one fit roughly on the "inside" of the area. (you sandwiched the metal plate below, your using this first layer to make up the "height" difference.).
Brush on more epoxy, make sure to soak in through all the fibers. Use another fiberglass weave, this time 2" over the edge, again, making sure to soak in the epoxy. Do this maybe another 2 times, and let fully dry.
This thing will be Ultra tough.
If your having trouble believing this will work, just take a few layers of fiberglass matt,, and soak it with epoxy. let dry, and tell me how strong it feels to you. /forums/images/graemlins/smile.gif