Dave , build a wood buck in the shape you want the grille to look like and build it ( the grille ) in 4 sections , Aluminum would be easier to use , just anneal it and form to the buck , then weld it altogether .
I think with using either metals you will need the help of a English wheel and a planishing hammer to get it alll smooth . Not unless you want to spend hours metal finishing with hand tools .
Good luck

Spray