I believe some kind of stretch warp could be a very reliable idea. Stretch wrap is a thin, stretchable plastic film that is used to lock and secure cased goods onto a pallet. As the stretch film is wrapped around the pallet, tension is applied, enabling the film to extend its length by up to 300%. This tension then creates a constrictive force around the load, allowing it to be held in place. If it were to rain there would be no water damage due to the wrap. This would ultimately be one of your best choices.