So, I've now added the rear diffuser I've had cut and ready. Finally figured out how to mount it to the car. I'd like to enclose the sides and also the top (around the same height as the top of the bumper...)
I had to cut space for the muffler and tailpipe, as well as the tow hook:
Mounted to the rear bumper cover:
Just recorded my best Tank AVG of nearly 63mpg. I cannot attribute all of those gains to the diffuser alone, but I certainly wouldn't have been able to hit that without it! (I don't think anyway...)