The process of making the heat-formed milk jug skull is straightforward -- you'll be using a heat gun to melt the HDPE plastic until it starts to become transparent, then press it into the details of the reference skull using a damp sponge. The sponge molds the plastic into the shape of the underlying form and cools it once it's in place.
Below are some process photos to give you the gist of it, then we'll go into more detail!
If you plan to fit the original skull onto a stand, you may need to cut a hole in the base first.
The original prop skull may have a movable jaw -- you can use a bit of hot glue to keep it from moving.
If mounting the skull, place a bit of hot glue on the top of the stand and then fit the skull onto it.
You can now fit the milk jug over the skull.
Remove Skull from Form
Use a hobby knife to slit the top of the skull toward the back and down.
Peel the new skull off of the form skull.
Next, we'll wire up the lights and sound!