If it's a fairly small ledge, you could just use layers of egg crate and shape them so it resembles a topographic map. However, this would get real expensive real quick for any decent sized slopes.
You can also build boxes out of them, and either glue them together or hold them together with zip ties. Some people use pieces of pvc pipe as columns, and a piece of eggcrate as a shelf.
I'd probably just go with lava rock, or stuffing a bunch of gravel in nylons or something.