Perhaps I misunderstood Max when he originally discussed the hemp. I assumed he meant using hemp fibres as a replacement for 'glass in some sort of chopper gun style pressed-into-a-mould kayak. The thickness-weight to strength ratio of such boats is deplorable. Your idea of replacing the cedar with paper mache seems a little shortsighted. Nothing, not even the myriad of poly foams currently used in modern yacht construction, compares to wood, ideally cedar or balsa, when used as the inner core or web in a monococque type fibreglass layup. I would spare the egg cartons and go with a straight fibreglass (or Kevlar) and epoxy layup and add braces or ribs after removal from the plug.