Well, I decided to add the extra balsa on the firewall before the firewall was attached. This allowed me more room and control to glue the scrap balsa to it and sand it to shape. I decided that I would glue the firewall to one fuselage side at a time. This allows me to make sure that I have a good bond and not create a mess in the process. Again, I am using 30 minute epoxy with milled fiberglass to give a strong bond. Once the epoxy has hardened (overnight), I will pin the firewall to that side. I made some "wood" nails to pin the firewall. I will put two on each side, staggering them so they are not in line with each other.