Getting enough skulls was another thing.
I had a real good dib into the spares kit part boxes and found the remnants of a visible man kit.
Took the skull (near as dammit the right size) and made a one piece rubber mould,
then came the amusing bit.

I knew I would have to make some of them hollow for effect so this is how it was done.
Since the mould was only going to have about a dozen or so uses then be thrown
away I didn't bother with being too precise.

The skull was cast with a 3mm hole at the back of the mould then
just slit down the bottom to get the original out.