Well you have plenty of material to practice tying it down now! If you try spread the moss quite thin and tie every 1/4" or so, going back over is fine. Where the moss is damaged by the tying it will grow a new shoot and attach to the wood or stone.
I like your sketch too, that would look nice. Vals would work - or maybe mossy ropes?????
My daughter's college 10 gallon tank had a large crypt with moss anchored with pebbles she had collected holding it down in it. Looked great. How she kept the moss so clean is a mystery to me but it was pristine. She pulled it up when it took over and put some back down. I think anchoring it helped the appearance as it didn't roll around. If you have a collection of pebbles you could try that too.