ORIGINAL: RCKen
An easy way to add strength to a firewall in a situation such as yours is to pin the firewall. Drill holes through the side of your fuselage into the firewall. Then use a small amount of epoxy and drive the dowel rod into the drilled hole. Cut the dowel off flush with the fuselage side.
Ken
That's a good idea, but what size dowel? The firewall I have is only about 8mm thick (have to go check, but around there - don't think it's much thicker) - if I drill a hole into the side the ply will split won't it?