Quoted from wolverinetuner:But if they were big enough, I guess one could be cut to make a "tube" to extend another one with packaging tape for the full cab length. That would allow for a foldable, easily brought along, "just in case" rain cover like I'm looking for. Now the question is whether an inexpensive large bag like that exists. It would have to have an opening much wider than a standard garbage bag.
I used 98 gallon medium duty clear cart liner bags from Our Shipping Supplies.com. A roll has 135 bags. Cost about $1. I cut them up to fit. I use a tarp and the bags.