Modeling clay works fine if you're going to paint it with latex, if you're going to use plaster using an oil based clay will work best since it doesn't dry. This way you'll be able to get the lay out of the mold easier.
Plaster of Paris works just fine if you can't get your hands on anything stronger. Plaster also comes in handy to make full or half face masks but that gets more in detail and I'm just no teaching myself how to do it. So that will be something to look forward to for next years Halloween videos.
Remember you DO NOT want to use your regular makeup brushes with liquid latex, it will ruin them!! Use the brush that comes connected to to cap of the product, or if it doesn't come with a brush use cheap brushes from the craft store (that you can throw away after) or sponge brushes.