3ft of 10"pvc pipe with the ends reduced to 6" to match ducting. inside it I lined with wall insulation and a 6" roll of chicken wire down the centre to keep things in place
beats dragging an old bath tub through the bush as a few of my mates have done before.
ive always dug a pit and lined it with silage film or similar. guess the airbed would cut alot of evaporation tho.