Well, I painted mine prior to filling it which doesn't help you much...but there are still other options, like using acrylic sheets and fastening it to the back, you can buy tank backgrounds by the foot at most aquarium stores (even Petsmart, Pets Plus, etc...). There is even a plastic material used for making signs I believe that is corrugated like cardboard that comes in a bunch of colors and is lighter weight (and probably cheaper) than acrylic sheet.
Chris
Chris