I think some of your terminology might be off. Are you talking about sculpting a mask in clay, and then pressing the wonderflex over that, or are you talking about starting with a mask of some sort, and then pressing clay around that to make a mold (aka, a negative)?
I wouldn't recommend just trying to copy a mask sculpture by pressing wonderflex over it. Due to the thickness of the Wonderflex, you will lose far too much detail; you'll have no sharp edges at all. It'll look like a Dr. Doom mask that has been left outside and covered in an eighth-inch of snow.
To do this in wonderflex, you'd really want to make a rigid negative mold and press the wonderflex into that.
Clay is a pretty lousy molding material. Especially when you need something good and rigid for thermoforming. After sculpting your prototype mask, you'd want to make a mold from plaster-of-paris. and then press the heated wonderflex into that. That way, so long as you made your mold properly, and you press the wonderflex into the mold completely before it sets, you will get an exact copy of your sculpture.
To improve your odds of pressing the wonderflex into the mold quickly enough, you'd want to pre-heat the plaster mold up to 175F. Unfortunately, if you do this, you run the risk of the mold shattering due to escaping steam or thermal shock. You gotta give the mold a couple days to dry, and then heat up the mold very slowly, and kitchen ovens aren't very good at this. You also want to encorporate burlap reinforcement into the plaster mold to help hold it together. That can mean the difference between a harmless hairline crack in your mold and a split right through the middle.