Easy peasy - I'll make a snake of polymer clay, make a ring of it, and push the mold onto it and that will hold it flat and steady while the concrete sets. No problem.
And because I had another thing I was making with the concrete - a ring, which I stabilized in an upright and flat position by cutting a hole for the ring band in a styrofoam meat tray, I put the polymer clay (un-baked) onto the styrofoam tray as well.
So, a week or so later, when I go to clean up (because it is unheard of for me to clean up on the day of!), I discovered this.
The unbaked polymer clay has reacted over time with the styrofoam and make a gooey mess. It ate a channel into the foam, and stuck to the polymer clay, and just thoroughly went icky.
I had to throw both the clay and foam out. No biggie really, but lesson learned. Don't leave unbaked polymer clay sitting on styrofoam for any length of time.