you could do it with eggcrate and cable ties. you have a flat piece of crate parallel to your tank back, and you can have one perpendicular strip running down the centre upright making a T shape. You would have to see how thick the middle bit of the eggcrate will have to be to hold it or you could have three uprights, and a big base, so if you are looking at the side of the tank it would be an L shape, the base you cover with sand, and the back bit you cover with your rock, or paint it black!! thats a good fix. but you would still need those centre pieces to stop it buckling in on itself. savvy?