Beware of adding too much reinforcement at the expense of a lot of added weight. Locking the corners of the firewall to the fuselage with triangle wedges is sufficient, trust the strength of the balsa and plywood and the basic engineering of the design. If you do decide to use fiberglass and epoxy, limit it to strips in the corners of the fuselage box structure. There's no advantage to fiberglassing the entire fuselage, it just adds weight. My downfall when building is too much reinforcement, I build strong, but I also build heavy and it showed in the plane's performance. Higher landing speed, higher flying speed (greater wing loading), all related to the added weight. Sometimes it required added weight up front to get the CG where it needed to be, compounding the weight issue. Granted, these planes don't need a lot of power to fly and you'd like them to fly well and even better, survive a bit of rough handling, but the added strengthening doesn't always translate to crash damage resistance.