Silicone casting question

I'm in the process of putting my Crash Corrigan gorilla suit together and having RPFer Rob (Daegurth) make the metal underskull. How would some of you experts sculpt and mold this so the finished silicone fits the underskull? From my research, those old gorilla costume heads were formed leather. Would appreciate some advice on this.

Hey there,
If it were possible to take a casting of the underskull (alginate would probably be the cheapest) to create a core, it would be what i would do. Core your mold with the underskull shape and then use your outer mold over it (making sure to use keys or some other way to ensure that the two molds line up properly. I'd use a high stretch silicone for this such as ecoflex or dragon skin by Smooth-on to give you the mobility you're looking for. You can then decide if you'll do injection mold or a pour in (I'd inject). remember to de-gas your silicone.

Thank you!!!