The original outer covering seems to be slightly rubberized cotton fabric, extremely thin. Methinks that if i remove enough goop and glue this space-age superthin blackout fabric on it, it will hold light. I am also not sure about opening the seam, maybe just stretching it to its unfolded state will do the trick. Maybe use a piece of styrofoam as a jig...
It would definately be a lot simpler to source another set of 9x12 camera bellows, probably even cheaper than making my own...but as I said, they are scarce, unfortunately.