sorry for the late reply - been on a looooong holiday !!!!
the wals were 2 thin pieces of plasticard stuck together, then i put a thin layer of greenstuff over them, then pressed on it heavily with a modelling file - although some rough sandpaper would give the same effect, then left it a bit to set before cutting the brick shapes into it
hope that helps, if i do anymore ill do some stage by stage pics so you can see what i mean ( i say if its more like when as ive got some more spiders coming soon - planning on making up a couple of swarm bases)