It can be done without draining the tank, it's just a lot harder. You basically need to make all the pieces and concrete them. I would do the back ground in serveral sections (3-4, or more since it is a large tank) that will fit together like a puzzle. After letting the concrete dry for a day or so, they will need to cure in a large tub for several days, if not weeks, in salt water with multiple water changes so that they will not buffer your tank when installed. Instead of being held in by silicone or adhesive you will hold them in with multiple high strength magnets (they will need to be strong to hold down all the foam). One set will be embedded in the foam, and the other on the outside of the tank. This may take some trial and error, but it can be done. Hope this helps, or at least gives you some new ideas.