I believe an easier way to constuct a 3-D background would be to buy thick styrofoam and carve it to the shape you want it then buy epoxy paint (i got mine from aquatic ecosystems) and paint the whole thing. this will give you a rock hard background to the shape you want. no gloves no goggles and no respirator.
Use sprayfoam insulation like the kind you buy at home depot. Spray a blob in the general size and shape of what you want. Let it dry. You then can use a sharp knife to get the shape you want. You can then use sand paper and a dremel to get an exact shape. Then paint it with potable drinking water tank epoxy paint. Some reef tanks in Europe actually use this spray foam to form their reefs.
