In reply to John Newbiggin:
A sheet of secondary double glazing film from the likes of Screwfix for about £5. Looks like clingfilm but is much, much more puncture resistant. Can't cut into it though without risk of propagating tears.
You can tie a knot in the corners (like a hanky hat) and peg out through them.
Because it is very light you'll need to be on your guard to prevent it blowing away and creating more litter in the countryside.