OK, I kind of hate to say this, but that firewall was wrong all along. You should always have something in those corners no matter how little power you think you have. Even some 1/8" square stock would have been good. I'd put some 1/4" triangle in place with epoxy. Be sure to sand the contact areas as best you can to get to clean wood for a good glue joint. An emery board or sandpaper on a stick will work pretty well.

Glad the spinning prop didn't cut any arteries. That tends to make a person's day a little less sunshiney.