Using leftover pizza boxes is a great idea for stepping stone molds. I order carryout pizza at least once a week, and this is a perfect way to recycle those boxes.
I want to make some personalized stepping stones from my garage to the side door of my house. When I looked online there were all kinds of helpful ideas explaining how you can do this.
It sounds like you can't go wrong with it. If you don't like the mold you created, it is easy and inexpensive to just make another one.
Even if they only last a few seasons, you aren't out much money and have had a lot of fun creating your own stepping stones for your yard.