This is the firewall on my Abbra---2layers of 3mm liteply with the outer layers' grain bonded at 90 degrees to each other, and 200 gram glasscloth bonded under pressure to the outsides. The captive nuts are held on exta little circles of liteply to avoid their tangs piercing the glass skin.
These sort of firewalls last indefinitely in my experience, used on20-30 cc 2 strokes soft mounted.
I have no experience of 4 strokes.
I have used carbon/end grain balsa laminates in the past---both sorts give a good wide (5mm plus) attachment thickness, but the glass/ply ones are equally light, and much cheaper to make.

PS the cut edges on the holes were fuel-proofed later!