They did an amazing Chaos Tree chained to some rocks in an old issue of White Dwarf. I think they used wire and filler to essentially make the tree and then did the rocks out of polystyrene I think. Have to find the issue.
How about an obelisk rising out of a crater shaped pit in the ground done up to look like a portal to the Chaos Realms? The edges of the pit could be writhing and mutating. I should do a visual plan of what I'm thinking about, it's hard to describe.





