Actually, I have used something like that. Last year I found a small box of something called "Magic Mix." It was a kids craft where you'd add an unidentified powder with a small amount of water, and within 45 minutes it would dry nice and hard. Basically, it was cement.
I used it to make a small handheld LED flashlight for a friend (mostly because I was bored and thought, "Hey! Concrete Torch!") It was...interesting to say the least. I even stupidly froze the linear switch and had to partially carve it out to free it.
The styrofoam/acetone trick is lots of fun. Take a tiny amount (an ounce to start) of acetone in a glass and drop in the styrofoam. It begins disappearing as soon as it hits the acetone. Pull it out the polystyrene goop that forms and you could probably do a lot of stuff with it, as long as you're careful about NOT igniting the acetone. This is the only way I can think of that you could use polystyrene since it decomposes from heat (which is why it isn't usually recycled), while PET can always be reformed from heat.