I have been trying to figure out how to make a background that I would like for quite some time. I finally came up with this. I used fluorescent light egg crating, aquarium safe silicone, and a bag of garden river rock from Home Depot. I had to cut the egg crate into 3 pieces to make it fit into the tank because of the glass support across the top middle of the tank. Probably just as well because the egg crating breaks fairly easily and with the weight of one solid piece I am sure I would have broken it before I got it in the tank.
Looking back I would probably use a sheet of 3/8" plexiglass cut into 3 pieces instead. (though it would cost significantly more). I have been really pleased with the way that it looks and my fish seem to really like it. They swim a lot higher in the tank now. It also seems to give the tank a lot more depth than not having a back. If you look in the bottom right of the picture, you will see a network of caves I made using the instructions by James Kuhn. Anyway here's the steps I used.