I'm planning on making the most movie accurate donnie darko frank mask as I can, so I decided I'm going to cast expanding urethane flexible foam in a two part silicone box mold since I found that is what they used in the movie. I was wondering how much silicone I will need, or if silicone is even a good option because of how big the mold is. I'm estimating the mold will need to be around 18x12x7. If anyone could tell me how much silicone that will take or it there is a better option that would be great. I should note that this is my first project ever, but since it is a harder task I have been preparing with many months of research, this is the only part I am not solid on.