unclear by looking at the page / description / etc but the flexible opening is attached to the wall plate itself - bought this hoping it may be separate so I could use the flexible opening portion for a dual-gang setup I have behind my tv - not a ding on the product, just a lack of clarity in the images / description (since most similar products are in 2 pieces the wall plate cover and the piece that screws onto the box)
I have two of these installed right now with three cables running through one and four through the other. Overall, they work fine. Installation was easy, I just used a file to square off the hole and ensure a tight fit. I notices where I have a thick cable hanging in a U it pulls the rubber a bit. So far it's holding up okay but I wouldn't recommend it if you have a lot of heavy cables.
It makes a huge difference with a wall mount TV if the cables are not either hanging down in front of the wall, or in a channel that hides the cords, but still makes it obvious what is there. While using a channel cover does organize the cords, hiding them inside the wall creates a clean look with nothing distracting from watching the show.