For those following, a quick update. I used tuft testing as described in Julian Edgar’s books. I started with a duckbill rear spoiler, then tried a modified shape, and then ended up going back to a deck lid extension, based on the Ford Fusion 999 LSR. That seemed to have the smoothest flow with the least separation. I went on a road trip and got almost 497 miles on the stock 13.2 gallon tank. Simple and functional. Read about it here:
https://ecomodder.com/forum/showthre...tml#post686638
It’s actually very tough (I like coroplast engineering), as I used triangular reinforcement gussets as well as a cross-beam at a right angle to the deck. Very lightweight.
This is still a prototype. I want to do the most I can to reduce drag while keeping it looking cool and being functional. The key factor is that I want everything removable when I sell the car down the road. Most of my mods will be restricted to the trunk/bumpers/wheels for that reason.