Thanks for the support guys!
I've started at a new job so I've had some downtime with this project, but I ordered material to mold and cast the skull, so hopefully I will start molding it this weekend!

I've never done any molding or casting before so any helpful tips are more than welcome!
Silicone/Mold material adds up quick. Be very careful or you go through your supply quickly. Make the box to hold your piece big enough that your mold will have some thickness to the edges. You don't want to have it fall apart after just a few pulls but don't make it so huge that you are wasting tons of rubber.
Resin goes farther than you would expect and you don't have to do it all at once. If you don't fill up your mold with resin you can make more anytime. It will cure to the already poured resin like it was one so don't make too much and waste resin.
Once the mold is done you can figure out how much resin you need using water. Won't hurt your mold and you won't waste resin. Nothing worse than having extra resin and nothing to poor it in. Take a measuring cup and figure out how much liquid the mold takes.
For the love of God and all that is good and holy, do not get so excited that you put your piece in your mold box, poor the silicone/rubber and then after realize you forgot to put the anti stick coating on the object thus making it impossible to peal your piece out of the mold cleanly. Your item will be fine but you will have wasted lots of rubber.
The mold will pick up every detail so the cleaner the master the less clean up work. Having said that don't drive yourself crazy trying to perfect the master. There will always be clean up.
Your item will "float" in the mold box once the silicone is poured so consider doing it in two parts, bracing it, hot glue an edge to the bottom of the box if that will work for the part, etc.
To get a feel do a small mold and resin cast of something tiny just to get a feel for the material.
This is based on limited experience as I have only been molding/casting a short time myself so if anyone has more or better ideas or thinks I am wrong about anything feel free.