I took a lifecast of the actor's face by pouring melted clay into the mold we made of him, and was able to carve the wounds and...weird stuff directly into it.
Then I molded that and painted in a few layers of a soft silicone (so that he would be able to emote somewhat through the mask).
Some paint...
Some slime (which was actually just more silicone so it would be totally solid and non-sticky)...
Tie on a couple elastic straps, and he's good to go!
It's not the best method, mind you, but for four evenings of work, it works pretty well.