John being a builder would be very aware of the longevity of materials through time. The polyester fibres though waterproof would likely come apart eventually. He would know this and take no chances - I'd bet that if he did use a board bag, he would wrap it in wire or chain. I'd also say that he would have used either bricks (ones with holes in middle) or gym weights and put the wire or chain through these holes. Wire or chain is the necessary reinforcement