1981 240D

I have the usual rust under the batter. Rain just pours into the passenger compartment. Yesterday I cleaned the entire area from the enging compartment and from the passenger compartment. I then put Rustoleum Rust Inhibitor all over everything, also from both sides. I am planning on fiberglassing everything and then topping off with undercoating.

I think the original drain hole my be rusted away. Any suggestions on how to approach this. Should I fiberglass over everthing including the original drain hole?

Any other tips or recommendations?