Have you looked at the GTI W12-650?
Flying butresses and an internal duct for downforce.
I made this design for a VW Beetle.
The dashed line inside the leading edge is a Dyson/Jetoptera type Coanda duct using engine cooling air to generate thrust. I have the Beetle but no way to fabricate one.
For sheet plastic I would look for a polymerized coating of some sort, possibly hydrophobic.