If you really want to do this right it’s going to cost you quite a bit.
To make a mold you need to find a cheap rubber ball or a giant charismas tree ball little bigger then your Skull.
On the ball or ornament make sure you fill the air hole smooth with something like epoxy glue for the plastic mold line to the rubber ball you can’t do anything about that at the moment.
The mold compound of choice is Silicone but Urethane is less expensive.
And please go through and watch the instruction videos they are indispensable.
For the silicone you don’t need a release agent except for where the two parts mold meets and make sure the ball is clean and try to line up the rubber ball mold line when you cast it with the silicone .
Also mask sure the casting hole at the top is at lest 1/2 wide in diameter so you don‘t pour the clear resin on the lip of the opening.
Clear-Lite Casting Resin (polyester) is the casting agent to use since you can cast in thicker layers but it might to get sufficient air and become tacky but EasyCast Clear Casting Epoxy is more of a plastic base agent it will dry smooth (not tacky at all )but you have to do more thinner layers.
The step is pour a small base layer into the mold without the skull in it for it to sit on.
Before you add the skull seal any holes in it with epoxy glue. Then glue the top scalp to the base layer of with epoxy glue. Then reposition it with you finger to the bottom of the mold again. Then slowly add layers thin layers till you filled it. The thinner the better since it will allow more air bubbles to escape.
When your done you should have only one seam line and bump from where the rubber ball hole was.
If you can get the giant ornament you could line up the opening of the ball with your pour hole so that you only have a one seam.
The likelihood that there prop department got in touch with a Acrylic(plexiglas) casting company to cast this bowling ball is high and a lot more effective but out of the scope for price to make the steel mold needed.
I would also contact the tap plastic people about this to get their input on this as well and make a copy of this for them to correct where it's needed.