It's the wrong way to do it.Period.
You'd be better off just slathering it in bondo. Yeah, still the wrong way to do it, but at least that way you're using a more standard practice even if it's in an amateur way.
Sorry, but fiberglass should not be used on metal. And that is bodywork 101. You don't fiberglass metal. And fiberglassing the back? So you can't figure out what happened with a magnet from the outside? Yeah sorry, that's shade tree, flip-job crap right there. If you have to conceal your "work" on the backside and cover it with interior panels you're doing it 100% wrong.
You're not going to convince me otherwise either. It doesn't matter if it would solve the problem. It's dirty pool. You're just screwing the car and the eventual next owner.