here are my thoughts. I use a pizza box to make stepping stones. I fill the box with dry quick-set to know how much to mix with water. I would add some caulk to the bottom of the box to make the grooves, and them put a large plastic garbage bag inside, accentuating the grooves. I think just crumpled plastic would flatten under the weight of the wet concrete. It depends on how deeply lined you want the surface to be. I would probably sift some of the course grains out of the concrete to make a smoother stone (less concrete looking-but some people use other products). You could always put some sand in the bottom of the box to make it less square. add in the wire coat hangers to reinforce it so it does not break apart (ask me how I know!). You could also press some leaves into the top to add texture there incase you do not like how the other side turned out. You can probably make two or three stepping stones from one bag of mix, so just try it.

Chris_MI, thanks for your input. I didn't get back here because I've been busy, but your advice is running along my thoughts.

Thanks for your time!