LOL yeh the pitiful detailing is my first plugging of "bondo glass". The builders ran the furring strips straight around the perimeter so the corner radius was an empty cavity.
The opening is not tight to the window frame by a long shot and thats what led me to use the resin so there would be less cavity to fill with sealant. After using it, I am kind of surprised at how well its doing the job. Its a very thick polyester resin with 1/2" or so hairs.
I agree with your stressed skin observation. Everything is glued to help make it all a unified assembly.