These pictures are of my rear diffuser. First off let me say I like how the Mopar rear diffuser looks though it's not a diffuser, it's more of an air blocker, it does not help air get out from underneath the car as a true diffuser would. I'm not saying what I have made creates any kind of huge downforce (and a true diffuser would), I do think it straightens up and aids the air flow under the rear of the car. It also fills that gaping void under the back of the car that already has a pretty decent diffuser-looking shape. The car also has an SRT front fascia and Koni shocks as the the only other non-stock items.

The material it's made from is called Tegris and it's 5/8th thick. I believe it's a type of modified polyurethane. I bought the material from roushyatesparts.com, it was a NASCAR COT splitter in it's former life and was only $40 used, though what I got had not been used and was unpainted.
It looks pretty much like carbon fiber, the gray weave is part of it though it's only on the outer surfaces, it works pretty much like wood and is easily painted.
The surface texture can be seen after it's painted. I painted the fins, the panels (behind the rear axle-in front of the rear fascia) and the indented area of the rear fascia around the exhaust flat black.

Picture A: This was a mock up for position and fit, the size of the fins got a lot smaller at their final size, raw material with unpainted panels.

Picture B: Close to final size and painted, with an additional fin outboard that is at the edge of the inside of the rear tire. Left side panel shown with the fins attached, right side panel (on the left in the picture) with the fin unattached and showing the 3M double sided adhesive tape that sticks (only the fins) to the rear fascia.

Picture C: How it attaches to the under side of the OEM rear panels, long sheet-metal screws and washers. Fins at final size, one attached, and the raw material unpainted.