If the base is Flat use a piece of thicker plastic it will out last the sled even in the wetest conditions.Also I believe it might be lighter then wood of the same thickness.Another option is spend a bit more jing and get a BOSS seat much lighter then stock.
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?
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