I deliver newspapers. (Yes, some people still do read them.) Anyhow, the driver's side window is almost always open and I'm subject to getting soaking wet when it rains. Any and all ideas would be appreciated and seriously considered.
This is not a hack, per se; but, there's a flip window attachment made for rural postal carriers that attaches to your car window without special attachments.
It's marketed and sold as a "Flip Window" by a company in the United States by the name of Postal Things, Inc.
You could make a similar kind of "awning" for your window that attaches by plasticized magnetic strips along the top of the window at the roofline. You would reach out from under the flap to deliver the paper.
You could tape a moderately-heavy (5 or 10 mil) piece of vinyl or polyethylene sheet over the window opening and cut straight lines across the center, vertically, horizontally and diagonally in a "starburst". If the sheet is stiff enough, you could drive at moderate sped and little water would get through, but you should be able to push your hand through easily.
N.B. Don't leave sharp points in the center, lest it cut your arm or cause it to get stuck pulling back! A little hole in the center should be acceptable.