[Bob Parmenter]

Towing open trailer with a cover on the car is a major bad idea.

As a variation on Turk's suggestion there is a large format shrink wrap available. It is a relatively heavy guage material, white in color. Boat manufacturers use it to cover their vessels while being transported on semi trailers. I've seen it, had a small sample that I've used for wrapping some parts, but don't have a source for you. I suspect you could contact a boat builder, or transport company to see if they can source it for you. Or do a Google search.