That's a good idea. I thought about driftwood and rocks, but I hadn't thought about using large flat stones as the support for the "carpet." I should be able to scrounge up a piece of slate somewhere.If it was me I would find large flat stones and tie the moss to it in a thin layer with cotton thread. Eventually the moss will attach to the stone and the cotton thread will decompose.
Then all you are left with is large flat moss covered stones.