foam is your best bet..can be easily cut or burnt to the correct shape you then cover it with papier mache to provide a key for the plaster or you can use polyester resin . then you can paint or throw sand/stones on it to give a more natural look..
you can slso use that insulating foam that hardens...the gap filler one..makes some funky shapes..only problem is it is toxic so you need to wash it about 25 times!!! to get the toxins out...(even then don't know if it would be too safe if eaten!)
you can use stuff like bamboo which is quite cool..just line it up along the back..cork sheeting is also good or cork bark (if you soak in hot water it will bend flat)..you could also look up fern root but this is very hard to find! (but if you do find some pm me!!!

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