Started working on the bottom today by taking off the stock rear engine diaper. After I took it off I realized why they did what they did; The main cat hangs VERY low!
The attachment points on the rear bumper for the diaper are much too high, the flow there would be separated even if I covered it with something smooth.
I think I will grab some coroplast, protect it from the cat's heat with aluminum foil, and then use some long bolts and nuts to let it sit lower than the stock piece. I might consider reinforcing it with some Bondo Hair. The idea is air from the engine compartment can escape from above the coroplast, and air from under the car can escape underneath without separation.
(excuse this gentleman's ridiculous bodykit and fake diffuser, my exhaust also happens to be on the other side in the same location). I will check with sign shops and TAP plastics to see if they have some suitable scrap material tomorrow.
I'm starting to give up on finding sandable polystyrene foam in a small piece, what do you guys think about using wood+spray foam for the spoiler? It'll be harder to work than foam and might have issues with expansion with humidity changes. Foam would also be much easier to work with for rear wheel fairings.