I made them out of cereal packet layered over with paper and PVA glue until the structure became hard. On top of the basic structure I added lots of smaller bits of card, like round the rim and round the base. Then on the inside I mixed up some PVAcrete and applied it inside the tub. When it hardened it was like rock.
Then to complete, I painted them with acrylic paint and when finished lacquered the whole thing with acrylic clear varnish. Surprisingly when all that is done the tubs are really very durable.