I spoke to a buddy of mine who works in the automobile refinishing industry (body work, etc.) and he was aghast. He said it's only a matter of time before that structure disintegrates and leaves shrapnel on the roadways. It's not unheard of to customize a fender by sectioning different pieces together, but the "proper" and safe way to do it is to weld it all together. Bondo is not structural, its' for finishing only. I bought a crusty old VW bug once that had bodyrot holes covered up with fiberglass matting instead of being replaced with sheetmetal patches and it all fell apart. Water gets in between the fiberglass and metal and starts rusting.
Best of luck.
