All my tanks are painted background, but I am thinking of making my own 3D background with Polystyrene. There are quite a few tutorials on the net, and I think they look great.
I found a seller on ebay that sells rigid black plastic in 3mm or 5mm thickness and cuts to size free of charge. Just siliconed it on, will let you lot know how its worked.
Edit:
As another plus, im now processing 5ppm of ammonia in 12 hours... Just waiting for the nitrite now, then some fish to look good against this background...
Silicone has dired a litte bit, put the hood and lights on and providing the glass is clean of algae (will put clean glass on to do list ) it looks good.
I put it on the outside by the way, incase anyone was wondering.