I have a TXTR-LITE panel in the back of my enclosure. It doesn't have the depth of a custom foam wall or aquarium backdrop, but it is still awesome looking. It's an ultra-thin natural stone veneer bonded to fiberglass to form a flexible panel. Very lightweight and can be cut with scissors. UV and water resistant.
http://txtr-lite.com/cart/index.php?cPath=25
The panels are rather expensive, but they have a small selection of damaged or flawed panels for a ridiculously discounted price (about $28 + $10 shipping). Panels are typically 2ft X4ft. I bought a black imperfect panel and it only had a small scratch and chipped edge, easily disguised for what I used it for.
Best background I could have hoped for and probably one of the cheaper things I bought for my little guy.