Alright...quick update on how I solved this problem. The inside height of my tank is 23 5/8". The width of the opening on the top of the tank is 21 5/8". I simply cut 2" off of the bottom of the background with a table saw, horizontally. NO VERTICAL CUTS!!!! WOOHOO. I siliconed the 2" bottom piece back onto the rest of the background and siliconed the whole thing to the back of the tank. You can VERY SLIGHTLY see where the two pieces join, BUT I am using pool filter sand for my substrate and it covers the seam. In other words...it looks perfect and it is rock solid! I will post some pics in a couple days once I get the sand in and my plumbing complete. Thanks to everyone for their help.