hi tansy-moon, I have made walking stepping stones directly in the ground by digging a shape of a large leaf out and at least 3 inches of soil depth placing cardboard or plastic,(cardboard is easier to remove from the edges) around the sides and bottom of hole. Pour the mixed mortar, smooth, then cover with plastic, using rocks along to hold the plastic down, (not on top). Your idea with a taped box will work, I would cover the whole box with plastic as well as tape, tree branch indentions would like great but remember where-ever there is an indention this will collect water and or dirt later, so don't set these to deep. Yes you will need to keep the stones wet for at least 7 days.
If you write 'stepping stones' in the search box here you may find lots of other ideas or instructions. Wendy

if you would like to see my large leaf stones follow the link below.

Here is a link that might be useful: {{gwi:66783}}

thank you so much EE, you gave me confidence. I will definitely find that search box and will check out your leaves. I made some leaves once and got rave reviews, but they shortly crumbled to bits, so I have a lot to learn.