I decided to scrap the inferior half of the mold shell which was too far gone to use. I set the silicone into the face half and drizzled plaster over the back to hold it in place. The plaster all the way around also lets me hold it while rotocasting without compromising the shape of the silicone.
Since most of my keys didn't survive the original separation, I decided to keep the back shell pretty thin and I'll just break it off when I'm ready to demold. The rubber bands are just there for added support.