I made my own.
This was my first attempt in making a rear diffuser. I like how it turned out, but I think it's too narrow. I was going to make one that went all the way across the rear bumper, but I'm abandoning the project and doing something else this year.
I'm doubt is does much, but I doubt the 3D Carbon one you're looking at does anything either. If this is the look you going for just go to the Home Depot and get some plexiglass. Make a pattern out of cardboard for the fins then cut out the plexiglass. I drilled and tapped the fins so the hole thing screws together. Then I wrapped the whole thing in 3M Di Noc.