So I built a diffuser out of an old Sears rooftop carrier. It was plastic and had a curve I could use, and it was going in the trash anyway. I did build some strakes, mostly for support. These are riveted on to provide strength. Looks a little like tuner bling but they add a bunch of strength and kept a good curve in it for transition.
I had to cut the rear bumper, which bothered me at first, but it was a parachute really. I cut nothing but plastic, so its integrity is not compromised.
The bumper is cut flush to the steel support beam. The diffuser angle is a little steep, probably 15 degrees or so, but I can flatten it by adding a fill in piece at the bumper. I will do some tuff testing once the full undertray is in to determine the proper angle.