Could you? Yes, but not for the “years” you mentioned. There’s a 130 gallons of why I wouldn’t try to use wood in that tank. I hate to be the bearer of bad news, but that’s an in-home natural disaster waiting to happen. Ply doesn’t have a smooth enough surface. It will likely leak from the edges within a short time. It also bends when pressure is applied, so you can expect a heavy flex outward, which will create further problems with your seals. You could coat the ply to water proof it, but one tiny scratch of a rock, gravel, or while cleaning, will give your tanks shelf life about one more half hour—the wood will saturate, and lose any rigidity it had left. You will never sleep at night if you put plywood as your back panel. I’m not about to tell you it absolutely CAN’T be done, because with enough experience and knowledge of the wood-working trade, people can do amazing things with the right equipment. BUT if you’re looking for an easy hardware store solution, I’m here to tell you wood is not the answer. A pane of glass that size and at least 3/8 inches, will probably cost close to 500 dollars, but you can order them online, or possibly at local glass store.