how to save deforming rubber mold without keys

I want to cast a statue of a Holy Mary, it is around 65cm tall and has a pretty "phallic" shape. For that I used silicone and made a brush on mold to be later supported with a mother mold and filled with plaster.

I am new to casting so I watched several tutorials and only after Ive applied my first layer of rubber I realised I needed to make keys for it. I didnt know how important they were, neither I had anything to pour them into, so I just decided to skip this step.

AFter making the rubber mold and plaster mother mold I removed the original Holy Mary from inside and I was ready to pour fresh plaster inside. However, when opening the rubber mold I cut it half way throught its height in a zigzag shape, so that I could remove the rubber mold. The problem is that now, when the rubber mold is placed inside the mother mold without anything inside, the zigzag keeps opening and the rubber deforms on the back of my Holy Mary. I tried to pour plaster in it but once I turn it around (as I want the statue to be hollow inside) the zigzags open anyway. The rubber in the zigzag is around 1inch thick.

Now, since the zigzag cut of the rubber mold keeps opening when being turned upside down I cannot finish my statute- is there any way to save this rubber mold and make the zigzags stay in place?

What if I first pour a line of plaster just on the zigzag opening and wait for it to completely solidify and than pour more plaster and start turning than? Would that work?

Thank you for any possible help!
Best

Natix