"I need a heck of lot more of these ..."
Nyuk, nyuk, nothing is ever easy. But wouldn't it be nice to have a big pile of these little skulls for you terrain.
The easiest stuff to use is Hydrocal or plaster of paris. Simply soften up some plasterscene (modeling clay) and push your piece into it. Fill it up with hydrocal and let it dry.
Not going to lie. This is the stuff the masters use. It works great. Costs lots. And they have a section on mold work. http://www.smooth-on.com/
Here is a vid using real old plastic from them in a plastercine mold.
Here is a link to an affiliate store to get Casting Supplies (canadian)
Collect lego blocks.
Tips and Tricks
Pour plastics from high in a steady, very thin, stream. This helps elongate the airbubbles and forces most out.
Would be nice to have one.