The Rubby cape was even tried in the WWII jeeps, but it was realized that it would make it difficult for soldiers to react quickly.
The air tight lap robe is still available, although they are sold as heavy duty garbage bags. We went camping in out 1942 GPW a few years ago in October, my wife was worried that it might rain, I assured her that it could not rain. It turned out I was right as it was too cold, but we did get snowed on. I put her in a couple of garbage bags, they kept her warm and dry (I did cut a hole for her head)