if you ask me, you're going to need 3 boxes behind the TV. One for the mount, I agree with SB Smarthomes on the Chief. Then you'll need one for the One Connect box. The Versa Box 20x14, or DataComm makes some good ones also. Neither of these will leave enough room though for a Sonos AMP, so you will need a third for the AMP and anything else you want to put back there.

It's going to look a little busy back there. Might want to think about doing a Sonos Port behind the TV and using that to create a source that you can send to an AMP in a different location. I think a Port will fit in a 14" wide box alongside the One Connect.