From the original post, I believe he is asking about the "in-tank" type of background. If he were asking about some sort of outside background, he would not be concerned about the leaching of chemicals. Cichlid fanciers (mostly) have tried multiple different ways of creating beautiful (and often very functional) backgrounds that attach directly to the inside back of a tank. They can be used to hide intakes, overflows, plumbing, etc. They also sometimes have caves and incorporate real driftwood. Most involve some sort of styrofoam/polystyrene material, coated with cement and/or epoxy, but the trick is finding the products that do not leach chemicals. One recent variety that I have seen involves that expanding foam-in-a-can stuff...used for filling cracks while insulating a house. The results are a bit unpredictable, but when painted are very nice! I think that technique involves a lot of practice first!
I will try to post some links when I have more time later...