Sounds like you're rebuilding a rather old seat as wood bases went out quite a few years ago, at least on Polaris. I've always replaced seats with dealer "take-offs" or else just bought the part of the seat I needed. The idea of pressure treated plywood sounds good. However, I'm not sure if it is available that thin and, more importantly, I wonder whether it will react with the aluminum beneath it. That is a moisture trap afterall, the reason the original needs replacing. And assuming your rig is quite a few years old, would not another piece of plywood of the original type most likely last the remaining life of your machine?