Massive rust hole

[Dimitalus]

Hey everyone I have a question, my truck had a rather large rust hole which I sanded down to bare metal and than sprayed I think it’s called rust mort, I was being shipped over seas for a bit so I also sprayed a anti rust paint on it ( the picture is before rust mort and paint) well I’m back now and I am not sure how to proceed with this I was thinking of using mesh wiring to form the shape some spray foam and than fibre glass. What I’m wondering is, what’s the best course of action here? Any help would be appreciated as this is something I’ve never done.(note the image below was before I finished sanding it down)