I think the last we heard from
Fred was in 2020 and that he was fleeing Shanghai and heading to Mongolia. Hopefully he's alive and well somewhere in Canada now. Anyway, he had a model and presented us with some pictures. You may find a trail to what you seek in his posts. I know that he discussed it with someone else who was interested.
Here's his main CFD thread:
http://986forum.com/forums/boxster-general-discussions/65306-boxster-986-aerodynamics-%40180km-hr.html
Here's his old Flickr album with a few pics he grabbed of the flow:
https://www.flickr.com/photos/gtcollection/