One alternative to using a denser metal over aluminum is to make firewall from aluminum and coat it with an intumescent. They react with flame heat and immediately expand into an insulating foam to protect underlying structure from heat damage. Engine nacelles on 727s used it effectively and oil rigs use it over steel supports. And I used it on glass sandwich cowl structure on my Pulsar. Did a test first and was amazed at the insulating qualities. But it does put out some smoke when activated.