UPDATE: after the fiasco with the first ebay setup, and the fact that I didn't have to pay for or return the first one, I bought a second one:
https://www.ebay.com/itm/223105891486
I have to say this one fits PERFECTLY. No gaps and all the tabs snap right in. It's gloss black and no carbon fiber though. I can live with that. Only issue is that the bottom holes of the diffuser, under the car, don't line up with the factory holes. So it's only held in by the two screws on the outermost portion of the bumper. Still feels pretty solid. I have a message into the seller for any advice but if you want a cheap way to add the diffuser, go for this one.