Whatever you do, do no just throw on plastic tarp, especially if the tarp is touching the bike. Condensation between the tarp and the bike will cause oxidation. Since the plastic doesn't breathe, paint will go to crap in weeks, rust will happen soon after.

So if nothing is coming into contact with the bike, plastic is fine. If it is coming into contact, a heavy canvas is fine as the water has a place to evaporate.