Other readers tipped packaging materials, such as wooden pallets, cartons and plastic stretch-wrap, as useful leading indicators.

The cube covers and card tray are a thin hard plastic similar to the plastic that box inserts are made of, so you might have to worry about splitting eventually.

To make his recycled paradise, Sowa packed bottles into plastic mesh bags and fishing nets. He lashed those to the bottom sides of wooden pallets, which form the island's foundation.