This is a modification to the car body itself, but it's the shape you want. It's not colorized but it's based off wooden boat decks with mahogany strips and brass screws, with body colored sides.
If you boat tail from inside to inside of the tail lights it saves a lot of fiddling with wiring & etc., but it induces separation before the boat tail. So — put [chevron-shaped] wickerbills or Gurney flaps at the leading edge of the taillights to wrap the airflow around.
What would really be humorous, would be to hang the license plate on a VW decklid for access to storage. 
edit:
https://www.greencarreports.com/imag...erbody-shields
Your really should start here. Compare to your 2011 undercarriage. Next....
The underside of your boat tail will look like this. A fenced diffuser.
Another approach is called a high-momentum mudflap. I've seen it used on some late cars but I don't have a good example. it's a converging rather than diverging duct with a different shape. It might fit a boat tail better, ending in a vertical slot on the lower tip. The closest I have is this:
Finally, rearward vision. The top of the boat tail can be a trough like you see on mid-engined car (DeTomaso Pantera, etc.)
https://wall.alphacoders.com/by_sub_...era+Wallpapers