3/4 will handle 3 x 4 Seaboard pretty easy!! If anything put one support across middle but I no think you have to.
My box covers were smaller, lets say 3 x 2?? and you never felt anything move!! and I am 320 pound stomping on them all the time!
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1986 V20 
Old Fishermen never die, we just SMELL that way!!
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