Popular amongst some migratory beekeepers that I know , for hive covers, is used concrete form board material, a heavy plywood. 
  Some of my friends don't use it because of the weight. But those that do use it don't need any rocks or nails to keep it on. 
  It's somewhat hard to get. See your local concrete form company. They resurface their forms and then have to get rid of the left over material.
   
  Mark



 
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