Using only about 60 cents worth of styrofoam a suitable core for the model can be made. This greatly saves on the amount of Plastalina needed cutting costs and also makes for a more defined finished model ; the core acting as the skull and giving underling support.

Start by forming a block of styro the approx. finished
size of your figure. Then using an electric knife & wonder cutter carve and shape the block into your finished core.

Note: ears and nose were left off the foam core.
Only the basic shape that a skull would give is necessary at this point.