Here's a thought - would one of these help until I can find a way to build a proper boat tail or kamm?
https://www.realtruck.com/bizon-truck-cab-spoiler/
I know it's designed for trucks, but it would definitely provide a cleaner separation on the airflow. And keep the snow off my back window - my rear wiper sucks. Doesn't matter what brand I put on, it just sucks. I get like a foot wide section that's clean and then it just smears stuff around through the rest of it's footprint.