Cut a hole to leave room for the foam to expand and spill out, then lined up the halves and taped them back up.
I ended up with three of these, partly because this is my first attempt, and partly due to the fact that it apparently takes anywhere from 3-7 days for the foam to entirely dry. And there are only 50 days left til Halloween. Thus...3 skulls should yield a relatively good number of skulls if I keep on top of finishing them. Will have to see how this goes. Bit late to spray them tonight--will attempt tomorrow morning.