I've read that regular Douglas fir boards lined with plastic can last 4-5 years easily, possiblely 6, so I would just go for cheaper untreated thick lumber.
My own beds, I used double hardie plank cement boards that are painted on both sides, they are 6 years old now, no sign of any decay. It looks like they will last a long time. They are no t as easy to work with as wood, harder to drill holes and cut.
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