Being an AutoBody Technician, You are on the right track. But I would use a fiberglass filler. That looks like an area that is under stress and gets beat on a bit. So I agree getting steel mess in to form a profile. Fill the profile and sand, fill, sand and you can get the outside looking real good. But I think that looks like plastic rather than a fiber glass, so there may be an adhesion problem. If you do a test spot to see if the resin will adhere, you will know what the next step will be. Either continue if it adheres, or you can FORCE adhesion by figuring out the area that you are going to cover, and then you drill several SMALL holes through the plastic so the fiberglass resin can bond between the front and back. Then feather out the seam close to a curved spot to make it more invisible. There can end up being a lot of sanding to get it perfect, but spend the time to make it right. You will feel better seeing a perfect job. After the outside is perfect, I would go on the underside and build up some heavy support to take the abuse. also don't be afraid to reinforce the entire underside where you can to prevent any further breakage. Once it is shaped, a nice coat of paint over the entire piece, and it will look good.
