Thanks for the advice. I dont think the idea of the plastic will work, the old edge of the fender was cut away both because of rust, and to make room for larger wheels, so the old inserts wont be going back in.
Right now i guess I'll Rust Bullet or POR everything, and then rhino the inside, unless there is some way to manufacture a liner to fit. I was kind of hoping to make a liner, but to be honest, I cant see how you could get the right shape in that oddly shaped area.
Why do you say "rust bullet > POR" (I havent worked with either)