I was inspired by the taillights on 90's Caddys for a kind of a partial boattail. I noticed the rear end on my Tempo is slightly angled inward, and I decided to extend that angle out to the edge of the bumper:
As you can see, I cut out a couple sections and covered them with clear packing tape so the brake lights can be seen from the side. I don't know if you legally must have at least some reflective red lighting on the side or not, but just in case, I didn't want it totally blocked.
I also added mini-spoiler like extensions off of the top and bottom edges to better detach airflow, but I'm not sure how much it will help.
I'll be testing the effects of this tomorrow.