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Ends are like sponges and fabric over them only will compound the expansion/ contraction problems. Just like your treated deck, reseal your ends after cutting and it will last much longer. Besides, the ends make first contact when loading, the other ends face forced water and dirt when driving.
Here in western Canada, I also believe it takes much longer to dry out any water in the bunks because it is so much cooler here. Boats are often stored indoors and unheated. You can often see mini-icicles hanging off wood bunkers.
I saw some park benches once made out of recycled plastic 2x4s. Slap some ski wax on top and you have lifetime bunkers. I'm going to look for a source... as long as I don't have to buy a cord of it just to try it out.
Thanks everyone.