Yikes. I can only imagine the horror you felt when it ripped. All that work that now has to be redone. Been there, done that. It ain't a good feeling.

The work looks excellent thus far, though. I have to tip my hat to your craftsmanship!

I wonder if you could use a more stretchy material for the gusset that would still look pretty close to what you had intended. I know there are some materials out there that look like leather but stretch generously. They tend to be thin, but could be doubled or tripled in layers to provide enough stiffness for functionality while still being stretchy enough for the inversion process.