I made my first Ultracal 30 mold tonight: the sunburst-ish plate over Xiahou Dun's sashes. It was surprisingly simple. The process didn't harm the craft foam at all (unless you count some of the prototype getting pulled apart because I was fruitlessly yanking at it until I got the brilliant idea of popping it out with a screwdriver, which worked), so I'm all excited to try it out on my claw. I was even able to sand most irregularities out of the mold, such as the very slight undercut around the edge that gave me trouble removing the foam in the first place. I'm glad I only waited three hours before popping out my original because I heard it isn't possible to machine Ultracal that easily after it fully sets (after at least 24 hours).
The mold is not perfect blah blah blah, but the edges look much smoother now, and I got rid of the dumb undercut, so w00t. And it actually looks (near) perfect unless you stare at it dead on. Me happy camper.