Try a different method or two.
Personally, I'd say to go about it in a completely different way.
First: I'd use PVA glue and a lot more paper than you're using right now. A quarter-inch layer will be strong enough, and if you mix the PVA/water ratio correctly, you'll manage to stick it all together fine. Just get a hairdryer on it to make sure it sets OK, assuming you have the time to do this at all.
Next: once the cast is complete, cut it off CAREFULLY. By this, I mean 'cut it off into two halves', unless one cut allows the piece to flex enough to slide off. In which case, lucky you.
After that all you need to do is add a few layers over the cut you made to seal it up, and you have yourself the perfect mould.