The most important thing is to keep the plywood dry. If you cannot move it inside allow for plenty of ventilation. I would not use bubblewrap as it is impermeable and likely to trap moisture aginst the ply/varnish. I have a wooden/ply boat and the biggest cause in deterioration of the varnish has been the penetration of moisture from the reverse side of the ply, eg. undersides of decks and inside of rear locker. It is important that these unseen areas are finished with varnish/epoxy or paint to prevent the ingress of moisture.