I've used MDO signboard for lids for many years.  It's heavy, but lasts
forever, and doesn't warp.  I cover my truck bed with used
signboard--salvaged from the large full-sheet commercial real estate for
sale signs.  It's generally well painted, and lasts again for many years of
hard use on a truck bed.  You just have to look at the message on the sign
year after year.
Randy Oliver

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