ORIGINAL: rainedave
I've gotten in the habit of drilling a couple of holes through both sides of the fuselage an inch or so into the firewall and epoxying hardwood dowels into the holes to pin the firewall in place. It might be overkill, but it hardly adds any weight and it's stronger than balsa tri-stock. After you sand the dowels flush and cover the model you'd never know they were there.
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I use round toothpicks (cherry sticks) with epoxy and insert them into predrilled holes through the sides and into the firewall. It doesn't take much to add a lot of strength to the firewall. Triangle stock is good too.
Ed Cregger