plastic film

There is a film which is quite heavy and durable. It goes under the name
visqueen. The type I am thinking of is used to cover things which have to
be exposed to the elements but need good protection. It comes in widths up
to 25 feet wide and the rolls are so long I don't even want to speculate as
to length. Truck drivers who pull flatbed trailers and haul the high long
lumber loads use it under the tarps to give a water tight seal. You might
try talking to your local lumber yard and see if you could get some that
just came off a load or buy a length off a new roll. The loads are
typically 8 feet high 8 feet wide and 40 feet long. So a lot of it is used
for each load.
Phil