To go a really cheap, and fully custom route, use styrophome. A lot of the guys with nicer displays on Cichlid-Forum.com have either nice expensive Dutch-made backgrounds (Austin Aquariums has one or two examples of these, in African sale tanks), or DIY styrophome ones. All you have to do is get your hands on some sizable styro, cut/shave/sculpt it as you please, then apply paint, water-safe (chemically) sealer/clear coat, and then use whatever means necessary to attach it to the back of your tank (so it doesnt fall forward, nor float). They end up looking very good, and very natural, if you paint it good. Otherwise, I'd just do a makeshift "background" by having the rocks (probably limestone) built up in the back, and cascade towards the front. Any random means of stacking, or careful structuring, should provide a decently natural look, and plenty of cavernous space for the fish to use.