What to use to cover up furniture in the rain?

I’m going tonight to look at a sofa I found on CraigsList. However, it’s supposed to rain not only today, but all week here. If we buy it, we’ll be taking it in the back of a pickup truck. We’re wondering what we could use to protect it (it’s microfiber) during the 15 minute car ride in case it rains. Preferably something cheap and something easily found around the house or at Lowe’s or something.

We have a roll of plastic, but I’m not sure how we’d get it to stay on the furniture unless we covered it with tape. Any ideas?