One of the newer plastic Spore Mines (middle ones here:
https://www.games-workshop.com/en-FI/Sporocyst) looks kind of spiky and rock-encrusted, so that could make a good basis, then sculpt some sort of sinewy or lash-whip style attachment on to a spare set of carnifex-scale arms (probably the smaller set of scything talons would be good).