Steve,
The firewall is (relatively) small, and hangs out some distance from the rest of the larger section of the fuselage. It is held at the top by some small square balsa stringers, which are subject to torsional twisting. The firewall itself has the die-cut punch outs, which further make the firewall less rigid. Higley's book on Bipes suggests using basswood (or other hardwood) spars behind the firewall, and using larger size than the balsa sticks as well. Use some solid plywood for the firewall. Take the die cut part and trace it onto the new solid plywood in order to cut the part. You could also just leave the cut out parts in the supplied die cut part and glue them in as well if you don't feel like cutting a new part. The results are a much more rigid engine mount that is not susceptible to torsional twisting. This sounds like it would be a worthwhile upgrade for those using a high torque engine such as the YS 1.20 or 1.40.