Thanks!
Here's the final touches on the rear diffuser and undertray panels:
I left an inch of coroplast off of the end of the bumper so the airflow separates cleanly. I put another panel on the side to seal off the open area under the trunk, and made sure to fold some of the material up so it doesn't have a parachute affect. The other side is open, though, since I need to leave the muffler in open air.