I have also seen 3d diy backgrounds made from styroform "painted" with quickrete (concrete). To make it more "natural", there is available concete coloring. According to what I have seen, once the concrete dries, it does not need to be sealed. It seems to me that it still should be soaked to make sure all chemicals leach out of it, but, then again... fish ponds are lined with conrete without having to soak them. Using quickrete involved few steps.


I have not tried either one yet... still in the researching/planning stage.