More bad news.
Turns out I'll need to add a few more layers of fiberglass around the screen. Turns out that balsa wood isn't quite as strong as I expected.
So up next I'll be buying some angle aluminum and fiberglassing it around the perimeter of the screen to give it some strength. I'm hoping that the screen will still fit in the back if I do this, but if it won't work out I'll jsut use aluminum sheeting to get a clean and flush edge around the screen.
Looks like I'll have another month left of work.